The dry-out easily occurs on high-aspect ratio microchannel sidewalls due to the decreasing of liquid film thickness. In this paper, the triangular microgrooves possessing the characteristic of evaporating meniscus were designed on the microchannel sidewalls. The heat sink consisted of 33 parallel microchannels, having a hydraulic diameter of 100 μm and an aspect ratio of 4. A platinum film heater and platinum resistance temperature detectors (RTDs) were integrated on the backside of the heat sink to realize uniform heating and precise temperature measurement, respectively. Flow boiling visualization experiments were carried out by high-speed camera in triangular groove-wall and plain-wall microchannels at mass fluxes of 148–490 kg/m2·s and inlet temperatures of 42 °C and 60 °C. The boiling curve, heat transfer coefficient (HTC), pressure drop, and two-phase flow boiling instability were systematically investigated to assess the flow boiling performances. Thin liquid film was observed in the triangular grooves during the dry-out process, compared to the dry-out in plain-wall microchannels. The oscillations of wall temperature, inlet temperature, and pressure drop were significantly suppressed in triangular groove-wall microchannels. Moreover, the earlier onset of nucleate boiling, improved heat flux, and HTC were realized in triangular groove-wall microchannels compared to plain-wall microchannels. Therefore, triangular groove design on the sidewalls is a promising solution to enhance boiling heat transfer and suppress flow boiling instabilities for high-aspect ratio microchannels.