A heat pipe (20) includes a pipe (21), a wick (22), and an operating fluid. The wick is fixedly engaged with an inside wall of the pipe. The operating fluid is sealed in the pipe and soaks the wick. The operating fluid includes a pure liquid, and a plurality of carbon nanocapsules (23) suspended in the pure liquid. The pure liquid can be selected from the group consisting of pure water, ammonia, methanol, acetone and heptane. Each carbon nanocapsule is a polyhedral carbon cluster (231), with a metal (232) having high thermal conductivity filled therein. The operating fluid has high thermal conductivity and uniform operation, and is recyclable with no pollution of the environment. The heat pipe has correspondingly high thermal conductivity.