In the present work, the radioactive solution of ۰.۱۰ Molar &sup۳;⁵S-aqua regia with half-life of ۸۶.۳۵ ± ۰.۱۷ days and specific activity of ۵۱۹ Ci/cm&sup۳; was considered as a source. First, the beta spectrum of &sup۳;⁵S was calculated. Then, by MCNPX code the effects of factors such as radius variation, height variation, type of substrate and type of side surface coating on the source efficiency were investigated. Also, by discussing the effect of minority carrier diffusion length, junction depth, geometric shape of P-N junction and doping concentration on the property of betavoltaic batteries, the present work concluded that the best parameters for betavoltaic batteries are the use of silicon and the radioactive solution of ۰.۱۰ Molar &sup۳;⁵S-aqua regia, thickness of the titanium coating around the radioactive solution of ۰.۱ µm and junction depth of ۰.۲ µm, spherical P-N junction, Nd (doping concentration of N-type region) = ۱.۰۰۰ × ۱۰&sup۱;⁷ cm⁻&sup۳; and Na (doping concentration of P-type region) = ۴.۵۰۰ × ۱۰&sup۱;⁶ cm⁻&sup۳;. Under these parameters the short circuit current, open circuit voltage and energy conversion efficiency are ۰.۹۲۶nA, ۰.۲۳۳ V and ۲.۵۱۳%, respectively. Also, the amounts of the short circuit current, open circuit voltage, and energy conversion efficiency were calculated for the titanium coating with thicknesses ۰.۵, ۱, ۲, and ۱۰ µm. It is not easy to build the titanium coating with a thickness of ۰.۱ µm. So, in the manufacturing process, the titanium coating with a thickness ۱۰ µm can be used. Since the relationship between battery parameters and activity is linear. To offset the amounts of the short circuit current, open circuit voltage and energy conversion efficiency, the activity needs to be increased about ۶.۵۲۱ times.