The atomic layer deposition (ALD) of silicon nitride using silicon chloride was simulated by density functional theory (DFT) calculation to compare different nitriding agents. We chose the NSiCl2*/SiCl*-terminated Si3N4 substrate as the adequate chlorine-terminated substrate. We confirmed that the thermal ALD reaction using NH3 or N2H4 is energetically favorable and found that the reaction with N2H4 shows an activation energy of 1.77 eV, which is lower than 3.30 eV of the reaction with NH3. In the plasma-enhanced ALD (PEALD) process using NH3 plasma, NH2− ion and H⋅ radical remove the Cl atoms of the Si3N4 surface to form active sites for the following reaction, –SiNH2* group or –Si*. N⋅ radicals in N2 plasma may not remove the Cl atoms because the recombination reaction is more favored. The PEALD process using NH3 plasma showed a growth rate of 0.09 nm/cycle at 300 °C, but the growth rate of the process using N2 plasma is negligible, which is in good agreement with our DFT calculations.