Eight different experiments have been performed, ranging from 8 to 20 days, with a primitive equation model consisting of five atmospheric layers and one oceanic layer to investigate the relative importance of radiative heating and deep cumulus condensation, orography and initial conditions for the development of the mean monsoon circulation from June to July in the 0–180°E, 25°S–55°N region. Two alternative initial states have been used, the first one based on the observed monthly mean June pressure distribution, and the second on the mean June zonal average pressure distribution. Large-scale condensation and actual land and sea distributions are included in every experiment, while deep cumulus condensation and radiative heating are always taken together. The mean sea level pressure distribution and low-level flow pattern produced by these simulations are found to be mainly determined by the diabatic heating distribution and influenced somewhat by orography, but are almost independent of the initial state. For example, the locations and strengths of the mean July sea level low pressure troughs over the Arabian Peninsula and north East Africa, along 85 and 110°E over the cast coast of India and Indochina, and the Somali jet and its eastward extension, are accurately created by all simulations which include radiative heating, whether or not the initial state is based on the monthly mean June pressure or the mean June zonally averaged pressure, or whether orography is included. However, none of these systems is produced when radiative heating and cumulus condensation are not included. The low pressure systems develop faster when diurnal variation of solar radiation is taken into consideration, apparently, mainly due to the more vigorous cumulus activity during the day. On the other hand, the precipitation distribution is found to be critically influenced by both radiative heating, orography and initial conditions. For example, without orography, maximum precipitation occurs in the coastal region of East Asia instead of the region around 100°E, 25°N found with orography, while the precipitation rate is greatly reduced both over land and over ocean without radiative beating.