Experimental research of subcooled flow boiling heat transfer of de-ionized water in a high-aspect-ratio, one-sided heating, vertical rectangular microchannel having a hydraulic diameter of 0.94 mm (5.01 mm × 0.52 mm) is carried out. The tested heat flux, mass flux, and inlet fluid subcooling are in the ranges of 0–30 W/cm2, 200–500 kg/m2 s, and 5–20 °C, respectively. A series of 24 existing correlations for subcooled flow boiling heat transfer are systematically reviewed and evaluated against the experimental data containing 328 points, which give significantly different predictions. Some typical conventional correlations such as Kandlikar, Shah, Chen, and Liu and Winterton predict well, among which the Shah correlation performs the best with the mean absolute deviation (MAD) of 10.45%. However, they all neglect the effect of fluid subcooling, especially in low-subcooling regime. Based on the Shah correlation, a new correlation for subcooled flow boiling heat transfer in the vertical narrow microchannel considering the subcooling effect is developed, which shows higher precision of prediction with the MAD of 7.87%.