This study aimed to determine the initial air pressure influence on the in-vessel composting for the biodegradable fraction of municipal solid waste in Morocco. For this purpose, representative composting mixture was prepared in which C/N ratio was 26 and moisture content was 70 %. The in-vessel bioreactor was designed and used specially to evaluate the initial air pressure effect on composting process in this study. Thus, daily changes of internal air pressure and temperature were monitored, and physicochemical properties of different composts obtained were also analyzed and compared. Experimental results showed a significant increase in internal pressure corresponding to the initial air pressure of 0.6 bar and a slight increase for the other initial air pressures. The initial air pressure which equal to 0.6 bar allowed maximum value of temperature and final composting product with good physicochemical properties as well as higher organic matter degradation and higher gas production. Composts obtained from experiments under 0.4 and 0.8 bar showed good maturity levels and may also be used for agricultural applications.