Heat pipe is a device which transfers heat from one location to another with a small temperature gradient. Application includes use of cryogenic heat pipe in cooling infrared sensors, laser systems, cryocoolers, thermal control large superconducting magnets and tumor surgery. This paper deals with transient behavior of cryogenic heat pipes with wire mesh and axial grooved wick using nitrogen and oxygen as working fluid. A special liquid nitrogen cryostat has been designed and developed for evaluating the transient behavior of heat pipes at 77 K when the condenser portion is connected to the cold sink externally. In this study, the start-up characteristic of heat pipes is experimentally investigated. © 2006-2011 Asian Research Publishing Network (ARPN).