The dissociation process of CH4/H-2 gas mixture during EACVD has been investigated using Monte Carlo simulation for the first time. The electron velocity distribution and H-2 dissociation were obtained over a wide range: 100 < E/N < 2000 Td. The variation of CH, dissociation with CH, concentration in the filling gas has been simulated. The electron velocity profile is asymmetric for the component parallel to the field. Most electrons possess non-zero velocity parallel to the substrate. The number of atomic H is a function of E/N. There are two peaks at E/N = 177 Td and 460 Td. The appropriate E/N is suggested to be 500-800 Td for low temperature deposition. The main diamond growth precursor is proposed to be CH, and CH3+.