By using a wide field stereo-microscope and a CCD video camera system，the axial liquid flow and dryout point（wetting height）in vertical rectangular capillary microgrooves heat sink were observed and measured under pure evaporation heat transfer conditions.An experimental study of the influences of microgroove geometric parameters and working liquids on wetting height was conducted.The results showed that the wetting height decreased sharply as heat input increased under pure evaporation heat transfer conditions.The microgrooves with a deeper depth，a narrower width or a higher microgroove density had higher wetting capacity.Methanol and ethanol had higher wetting capacity than distilled water in the low heat input cases.A serious negative effect of gravity on liquid wetting characteristics in vertical microgrooves was observed.