Microgravity experiments on square two-component heat pipes made of Pyrex glass were carried out by utilizing the parabolic flight of an airplane for investigating Marangoni flow. Temperature variation in the heat pipes accompanying change in gravity level was principally dependent upon the distribution of working liquid. Although the liquid fill charge in the cooling section was increased abruptly by intermittent boiling, it later decreased at an almost constant rate except in the case of a relatively high concentration of initial mixture, such as 50 mol%. Comparison of liquid flow rates calculated from image data for the cooling section and liquid flow rates estimated from previous theory confirmed that the flow-back of working liquid induced by the Marangoni effect occurs by the predicted mechanism. It was also shown that the mechanism by which the working liquid is attached to the container wall was able to operate, and reformation of a liquid plug could be prevented, by use of square heat pipes; and that the application of the Marangoni effect is effective for generally used micro heat pipes.