Abstarct: The amount of stored energy in per unit of a substance mass depends on different factors such as the type of substance combining constituent, rays energy, exposure mode, exposure time and the size of the exposure area. One of the important parameters in addition to the above mentioned cases is computing this amount of energy in dimensions of milli, micro and Nano meters that has many applications in medicine and radiation protection. Dosimetry in small dimensions is called microdosimetry. The imparted dose distribution in produced material of distinct ray according to single event is being investigated in microdosimetry. Actually due to investigating environment’s small volume it can be assumed the imparted produced dose is just an interaction. Thus different techniques should be considered for microdosimetry and new quantities should be introduced. And since tow third of human body is composed of water and according to its density, it is close to body tissue density, water has always been known as one of the materials which has been studied for dosimetry. Therefore, in this work the dose distribution function was calculated of resulted gamma from 192Ir source in water with mentioned dimension and the effect of dimension on this distribution. And we realized that with the increase in dimensions, dose distribution function significantly changes and the left dose will decrease. Also achieved dose was compared to the result of MCNP code and we realized that the MCNP results for small dimensions will not be reliable. Truly by considering the inefficiency MCNP code in small dimensions, it cannot be used in microdosimetry simulation. since in MCNP tally or results are obtained by averaging. Because of this reason MCNP cannot consider single event, and since in microdosimetry we face single events, MCNP is not a suitable code for microdosimetry simulation. Therefore, in this we act to write a Monte Carlo code to obtain single event dose distribution. Finally, the impact of energy and dimension were checked on the dose distribution function. The results showed dose distribution function has a maximum value which is caused by maximum amount of energy imparted by electron range. In addition, depending on the specific range of electrons peaks are observed in dose distribution diagram baxsed on the lineal energy that are related to produced electron of Compton and photoelectric interactions. By increasing energy, dose distribution function will shift toward smaller lineal energy that this changing process had a great compatibility with others ‘work. But by changing the investigating volume dimension, due to electron range which is smaller or larger of desirable diameter volume, dose distribution function respectively moves toward larger lineal energies or it remains constantly.