A quantitative analysis of critical heat flux (CHF) in upward saturated two-phase flow in vertical rectangular channels under atmospheric pressure was successfully carried out by applying the completely separated two-phase flow model to the existing model of vapor blanket over the thin liquid sublayer on the heated walls at the CHF point. The analytical results give good predictions of the existing experimental results in vertical rectangular channels with 1.03 to 5.0 mm channel gap, 40 to 51 mm channel width, 350 to 750 mm channel length and 20 to 88°C inlet subcooling, and identify the effects of predominant parameters on CHF in upward saturated two-phase flow in vertical rectangular channels.