The particle size distribution of cigarette smoke is an important factor in predicting the deposition fraction of the inhaled particles in various regions of the respiratory tract. Mainstream cigarette smoke is a direct concern for smokers, while the exhaled and side stream smoke contribute to passive (second hand) smoking. The particle size distribution of tobacco smoke for mainstream and exhaled smoke has been studied in this work. This study investigates how smoking behavior, including puff volume and number of puffs, affect the particle size distribution of mainstream cigarette smoke. Scanning Mobility Particle Sizer (SMPS) was used to measure the particle size distribution of mainstream and exhaled cigarette smoke. The values of count median diameter (CMD) mobility size for the first four consecutive puffs were found to be 193 nm, 198 nm, 194 nm and 186 nm. As the volume of a puff increases from 35 mL to 85 mL, the CMD shifted slightly towards the higher particle size ranges, because of increased probability of coagulation and other combination processes. For the exhaled cigarette smoke, the growth factor was found to be 1.5 ± 0.3 with respect to mainstream cigarette smoke. The experimental results of particle size distributions were used in a Multiple Path Particle Dosimetry (MPPD) model to predict deposition patterns in the human respiratory tract. The MPPD model results show the deposition fractions for mainstream cigarette smoke were 0.163, 0.152 and 0.298 for the head, trachea and bronchi (TB) and pulmonary region, and those for exhaled cigarette smoke were 0.273, 0.064 and 0.134 respectively. The total calculated deposition fraction for mainstream and exhaled cigarette smoke was found to be 0.613 and 0.471, respectively.