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Flow visualization was conducted for the closed-loop pulsating heat pipe (PHP) using a charge coupled device (CCD). It was observed that during the start-up period, the working fluid oscillates with large amplitude, however, at steady operating state, the working fluid circulates. The direction of circulation for the working fluid is consistent once circulation is attained, but the direction of circulation can be different for the same experimental run. Phenomena such as nucleation boiling, coalescence of bubbles, formation of slug and propagation of inertia wave were observed in the closed-loop PHP. The findings showed that the meandering bends, uneven slug and plug distribution and non-concurrent boiling at the evaporator contributed to the driving and restoring forces for fluid circulation and oscillations.