Chemical vapor deposition of monosilane between two 4-inch wafers with 5 mm wafer spacing is simulated at molecular level under 973 K and 0.1, 1.0, 10 Torr. Four gas-phase reactions are considered, with a detailed reaction kinetics simulation on the first decomposition step of SiH4 to yield H-2 and highly reactive SiH2. Also four surface reactions of SiH4. SiH2, Si2H6, Si3H8 are included in simulation. Monosilane, with its lowest sticking coefficients among film-forming species, is the major reason for thickness uniformity over large area in silane CVD. Si2H6 and SiH2, both of higher sticking coefficients, are the contributors of film thickness non-uniformity. The simulation results indicate an interesting role switching between disilane and silylene as SiH4 pressure varies from 0.1 to 1 Torr. When SiH4 pressure is low, SiH2 has a higher probability to impinge on the wafer surface and deposit. Its contribution to deposition rate is higher than that of Si2H6; On the other hand, when the pressure is high, SiH2 tends to insert itself into SiH4 or Hz to generate Si2H6 or SiH4. Therefore disilane becomes the primary factor of non-uniformity, instead of silylene.