Abstarct: Ion traps have been widely used to trap ions. These trapped ions are ideal systems for study in various fields in physics. Some applications of ion traps include precision measurement of atomic masses of stable and unstable isotopes, atomic transition frequencies, lifetime measurements of mextastable excited states and their use as a mass spectrometer or an ion storage source. Ion traps are divided into two groups: Paul traps and Penning traps. Paul trap uses periodic quadrupole electric field and Penning trap uses the combination of static magnetic field and electrostatic potential in order to confine particles. In this thesis, we study a linear Paul trap as a mass filter. So at first, stability diagrams for ion trajectories in Paul trap are plotted with simulation softwares and the effect of magnetic field on stability parameters is studied and by using stability diagrams, stable and unstable motions are calculated and the best position for isotope separation is obtained. The results is verified and compared with simulation of ion motion in Paul trap performed with CST STUDIO. We compared all stability regions and possibility of isotope separation in any region. Then we found the best region for isotope separation of 10B and 11B is stability region “ C ”. If a constant magnetic field in y direction is applied, stability regions change and shift therefore the most suitable region for isotope separation is stability region “A' “.