Two-phase flow excursion instability especially for a downward flow in a narrow rectangular channel heated on both sides was experimentally investigated. Three test channels with different gap sizes (2.5, 3.3 and 4.1 mm) for the same channel width (40 mm), the heated width (30 mm) and the length (350 mm) were adopted for the experiment. For several imposed heat flux values (ranging 0-1000 kW m(-2)) and inlet subcooling temperature (43-75 K), subcooled flow boiling was investigated while reducing the fluid mass flux starting from a sufficient high-mass-flux condition up to the flow excursion point when vigorous boiling occurs. The minimum mass flux conditions in which a stable flow is sustainable were identified via 47 flow instability data points and were compared with relevant correlations. The results implied flow excursion points that were close to the onset of a significant void. The visualization of the flow excursion using a high-speed camera was achieved, clearly demonstrating that the flow excursion is triggered by the coalescence of facing bubbles (for Pe < 14000) or wavy vapors (for Pe > 14000) on opposing heated surfaces. Deviation from a prediction of earlier correlations with the present data was revealed, and new empirical correlations that reflect the gap size effect which predicts our data with fairly good accuracy are suggested. (C) 2013 Elsevier Ltd. All rights reserved.