This paper reports on a visualization study investigating the characteristics of the liquid–vapor interface behavior on the surface of a wick in the capillary evaporator of loop heat pipes. Observations were conducted with a microscopic infrared camera and a microscope. Nine different samples simulate a part of a wick were made by two different materials: polytetrafluoroethylene (PTFE); and stainless steel (SS). First, three types of liquid–vapor interface behavior are reported. They are (a) evaporation at the menisci that formed at the boundary line between the heating plate, wick, and groove, (b) evaporation at the surface of small liquid bridges through nucleate boiling at the contact surface between the heating plate and wick, and (c) evaporation at the menisci in the wick. Secondly, the effect of the groove width on the maximum heat flux and the depth of a vapor pocket in the wick are reported. It was found that, as the groove width reduced, a higher heat transfer capacity was observed in both of the materials, except in the wick that had the smallest groove width. Additionally, it was found that the wick with the smallest fin width had the greatest potential for preventing vapor pockets formation in the wick.