Parametric effects at critical heat flux(CHF) have been measured in subcooled water flowing horizontally in a uniformly heated tube under low pressure. The parametric effects of mass flux, inlet subcooling, ratio of length and diameter (L/D), and heat flux were investigated. A visual study gave helpful information on the flow structure at the conditions under investigation. An empirical correlation is proposed to predict burnout heat flux for subcooled flow boiling inside uniformly heated horizontal tubes. The effects of principal parameters are assumed to be independent varialbes without coupled each others. The proposed correlation agreeds with obtained data with in $\pm$ 15\%. The results show that increasing the mass velocity increases CHF for subcooled conditions, the increased inlet subcooling increases CHF linearly and CHF decreases as L/D is increased to a limting value (L/D= 15 to 80), above which it has a minor influence. At the entrance region CHF($L/D < 80$) is almost inversely proportional to L/D. The test section was destroyed by local burnout within a second after reatching the critical heat flux. In this investigation the exit quality of the vapor is always negative, however, pressure drops in the test section are smaller when burnout is taking place than when liquid phase only flows in the channel since frictional pressure drop reduces by local boiling. A theory of pressure drop and heat transfer for various flow regimes of two phase flow was summarized in chapter 2 and 3. Experimental data were obtained with water for the ranges of mass velocity of $0.8\times10^6$ to $4.0\times10^6$ 1bm/hr - $ft^2$.