The objectives of this study are to investigate the effects of the power transient on critical heat flux (CHF), and to develop CHF and maximum heat flux (MHF) correlations in transient boiling systems with a countercurrent flow between the liquid and vapor flow: The test section consists of narrow: vertical, rectangular channels between parallel plates. Rate of change of wall temperature at the CHF point, (dT(w)/dt)(CHF), and a nondimensional transient parameter, tau(o) = (L-2/alpha(f) T-sat)/(dT(w)/dt)/(CHF) are introduced to evaluate the effects of power transients on CHF and MHF, Experimental ranges were 738.0-1,968.0 kg/m(2)s for mass flux, 7.0-17.5 degrees C for inlet subcooling, and 3.0-8.0 mm for channel gap distance. The system pressure was kept constant at 1.0 aim. The experimental results show that CHF and MHF values increase with an increasing rate of change of wall temperature, and the increasing rate of CHF is higher in a wider channel gap distance. CHF and MNF increase linearly with increasing mass flux at the top of the test section, and the linearity decreases at the bottom of the test section, The effect of the inlet subcooling is significant at the top of the test section, therefore the test location closest to liquid inlet has a higher CHF value than the location immediately below it, It,was also found that the wider the channel gap distance, the higher was the CHF value obtained for a given rate of change of wall temperature. New CHF and MHF correlations are developed for countercurrent flow in transient boiling systems. The CHF and MHF correlations are in good agreement with the experimental data within +/-25% error bands, respectively.