The behavior of the interface deformation of a liquid film and the plug formation in a horizontal square pipe under microgravity was observed using the drop-shafts of the Microgravity Laboratory of Japan (MGLAB) and the National Institute of Advanced Industrial Science and Technology (AIST, RIIF). The shape of the liquid meniscus attached to the lowest solid edge of the square pipe became unstable due to the transition from normal gravity to microgravity, and the liquid film broke into several lobes periodically. Liquid plugs were formed under microgravity in the pipe. The experimental results of the average wavelength of the lobes spaced along the pipe were in reasonable agreement with the numerical results of the linear stability analysis for the circular pipe.