The purpose of this study is to conduct the critical heat flux (CHF) correlation for narrow channels, The CHF of subcooled flow boiling of water in narrow rectangular channels under atmospheric pressure was measured parametrically. Experimental test channels were rectangular and heated from one side with the channel gap of 0.2-3.0 mm. channel width of 7-22 mm, and heated length of 50-200 mm. First, the CHF correlation for the one-side heated rectangular channels was proposed with investigating the various system parameter effects on CHF. Next, applicability of the correlation to both-side heated rectangular channel, half-circumferentially heated tube, and full-circumferentially heated tubes was examined. New CHF correlation for narrow rectangular channels and small-diameter tubes was proposed using the critical quality, dimensionless CHF parameter and heated perimeter ratio. Calculation accuracy of the correlation is 45% (maximum 10 times better in comparison with the existing CHF calculation methods which were proposed for the full-circumferentially heated tubes).