The present study is focused on studying the combustion characteristics of the modified pressure-swirl atomizers (PSA) to operate as air assist pressure swirl atomizer (AAPSA) under different operating conditions and different sizes of the atomizer swirl passage (width x depth). The operating condition studied are air to fuel mass ratio which changed as 28, 37, 52 and 65, assist air mass flow rate which varied as 2, 4, 5 and 7.5 g/s, the input thermal load which changed as 28, 44, 60, and 80 kW. The atomizer swirl passages sizes are varied as 1x1, 1.5x1.5, 2x2 and 2.5x2.5 mm2. Combustion test rig is designed and manufactured to investigate the effects of the above parameters on the combustion characteristics. The temperatures maps, dimensionless visible flame length and combustion species concentrations along the combustor are measured and presented using commercial diesel oil as fuel. The results indicate that, by increasing the swirl passage size, the high temperatures region size increased, the visible flame length increased, CO and O2 concentrations increased, while NO and CO2 concentrations decreased. The axial NO concentrations trend is matched with the axial flame temperature profile. The flame length increased by increasing assist air mass flow rate, thermal load, swirl passage size and by decreasing the air to fuel ratio.