With the object of clarifying the fluid behavior in pool boiling at high heat fluxes on a vertical heater surface of comparatively large height, a study has been conducted with a simulated method employing a rectangular nozzle (100 x 10 mm) stretched with a 600-mesh gauge at the exit section through which air is discharged into the surrounding water. First, the propriety of this method to simulate the fluid behavior in pool boiling at high heat fluxes is examined carefully, and then the sequential state of fluid behavior is observed with video for both horizontal and vertical positions of this rectangular nozzle, providing effective information throwing light on the mechanism to cause CHF on a vertical heater surface of comparatively large height.