Investigations have been undertaken on the process of boriding by pulsed-DC plasma-assisted chemical vapor deposition technique (PACVD). The substrate material was 42CrMo4 steel. Boriding agent was trimethylborate [B(CH 3 O) 3 ]. The flow rates of Ar and H 2 were 2700 ml/min and 540 ml/min, respectively, the total pressure in chamber during plasma boriding process was 800 Pa, the boriding temperature was 830 o C, the boriding time was 1 h and the flow rate of B(CH 3 O) 3 was from 0.1 g/h to 1 g/h. A mass spectrometer was used to detect the ionization of B(CH 3 O) 3 . Experimental results showed that the boronized layer could be obtained under this testing process and the thickness of boronized layer increases with increasing the flow rate of B(CH 3 O) 3 . On the other hand, the B-C layer would be formed on the boronized layer when the flow rate of B(CH 3 O) 3 was higher than 0.5 g/h and the thickness of B-C layer also increases with increasing the flow rate of B(CH 3 O) 3 .