The critical heat flux (CHF) of subcooled water flow boiling for a high length/diameter (L/d) region is systematically measured for the flow velocities (u = 6.93 to 13.32 m/s), the outlet subcoolings (Tsub,out = 12.5 to 113 K), the inlet subcoolings (Tsub,in = 45 to 148.7 K), the outlet pressure (Pout = 773.50 to 861.12 kPa), and the inlet pressure (Pin = 796.16 to 920.07 kPa). Type 304 stainless steel tubes of inner diameter (d = 2 mm) and heated lengths (L = 21.5, 79.45, and 149.7 mm) with L/d = 10.75, 39.73, and 74.85 are used. The CHF correlation against outlet subcooling including the effect of L/d already presented by the authors describes the CHF obtained in this work within a 15% difference. However, the correlation against inlet subcooling also presented by the authors in the same papers needs a small modification to describe the CHF obtained in this work for a high L/d range. The modified correlation describes not only the experimental data for L/d up to 75 on the 2-mm tube but also the CHF for the same range of L/d on larger diameter tubes predicted by the correlation against outlet subcooling within a 15% difference.