Abstarct: In this thesis the electrical and magnetic properties of manganite samples with nano- and micro sizes particles prepared by sol-gel method were studied. The work was performed in two sections. Section 1: The bilxayered manganite LaSr2Mn2O7 samples with grain sizes from 150 to 1000 nm were synthesized using sol-gel method. After optimization of growth conditions, it was revealed that the samples synthesized at pH= 7 are single phase and the minimum temperature for getting the pure phase could not be less than 1250 oC. The phase formation studies as well as systematic structural investigation showed that the samples have a Sr3Ti2O7 structure type with I4/mmm space group. The magnetic measurements versus field and temperature on samples with different grain sizes indicated formation of the Antiferromagnetic phase by reducing the temperature which is accompany with charge ordering state. The phase transition temperature did not show any considerable size dependence but a broad transition was clearly observed for nanosize sample. Increase in saturation magnetization of the samples with smaller grain sizes was observed. Resistivity measurements with temperature were performed and the results were fitted by theoretical conduction mechanisms. Also the effects of a wide range of Co doping on structure, transport and magnetic properties of bilxayered manganites LaSr2Mn2O7 were investigated. The magnetic and electric studies on low doped samples demonstrated that the charge ordering transition temperature shifts to lower temperatures and the magnetization decreases with increase in doping levels. The heavy doped compounds were studied for the first time and the results revealed suppression of charge ordering and change in magnetic behavior of these samples. The magnetic measurements indicated the possible formation of ferromagnetic clusters and short range ferromagnetic phase for the heavy doped samples. Section 2: In this part, the grain size dependence of the magnetic properties of the La0.6Sr0.4MnO3 samples was studied. This compound has a ferromagnetic phase at room temperature. Structural analyses showed the samples grain sizes are from around 20 nm to 100 nm and the samples have rhombohedral structure with space group R-3C. The magnetic investigation of the samples indicated that these compounds are soft ferromagnetic and the Curie temperature shifts to lower temperatures with grain size reduction. The calculation of effective magnetic moment of the samples baxsed on Curie-Weiss law showed that it decreases with reduction of the grain size. Decrease in saturation magnetization was also observed with grain size. The observed behavior was investigated according to the core-shell model and the magnetic dead laxyer thickness estimated to be about 1nm for 20nm grain size sample. The investigation of magnetic dynamic behavior of the samples baxsed on well known phenomenological models, Nee-Brown, Vogel-Fulcher and critical slowing down models, indicated a strong interaction between particles. The Blocking/Freezing temperature displacement in nano grain size sample and their frequency dependence on ac susceptibility measurements revealed possible existence of superparamagnetism and super-spin-glass state in nano grain size samples. Magnetocaloric studies were also performed and the results demonstrated variation in isothermal magnetic entropy as well as adiabatic temperature parameter with grain size but the relative cooling power is constant. According to the chemical stability, simple preparation, low cost materials and control on transition temperature of this compound and baxsed on magnetocaloric studies, the LSMO could be considered as a good candidate for magnetic refrigeration above room temperature at moderate magntic fields.