The optical emission process of atomic hydrogen (Halpha, Hbeta, Hgamma), atomic carbon C(2p3s2p2 : lambda = 165.7 nm) and radical CH(A2delta --> X2II: lambda = 420-440 nm) in diamond film growth by electron-assisted chemical vapor deposition (EACVD) from a gas mixture of CH4 and H2 was studied by using Monte-Carlo simulation. The variation of the emission lines with gas pressure (0.1-12.5 kPa) of different substrate temperatures (573-1 173 K) was investigated. And the results show that at different substrate temperatures the intensity of all the emission lines increase with increasing gas pressure at first, whereas decreased afterward. Furthermore, the emission lines intensity decreases with increasing substrate temperature at a relative low gas pressure, while increases with increasing substrate temperature at the higher gas pressure.