Commutation failure is one of the most common faults in HVDC transmission system, which is affected by many factors. This paper analyzes the effects of some major factors, such as voltage of commutation bus, DC current, advance firing angle, short circuit impedance and ratio of converter transformer, on the commutation failure in the inverter in HVDC transmission system. The obtained general rule of commutation failure in the inverter in HVDC transmission system is helpful to practical operation.