Summary: A theoretical analysis is conducted to determine the primary factors affecting the startup characteristics of a pulsating heat pipe. It is found that the wall surface condition, evaporation in the heating section, superheat, bubble growth, and vapor bubbles trapped in cavities at the capillary inner wall affect the startup of oscillating motion in the pulsating heat pipe. The required superheat and heat flux level for the startup of oscillating motions in a pulsating heat pipe depend on the cavity size of the inner wall surface and the naturally-formed vapor bubbles and their shapes. When the capillary inner surface is coated or fabricated with cavities or roughness, the pulsating heat pipe can be readily started up. And it is found that the working fluid significantly affects the startup characteristics of a pulsating heat pipe. The results presented here can result in a better understanding of the startup operation of a pulsating heat pipe.