The purpose of this paper is to know the effect of Adjustment factor (C) in Penman equation In the modified Penman equation by Doorenbos and Pruitt (1977), Potential Evapotranspiration(PET) was calculated in cases of (1) neglecting Adjustment factor (C=1, 0, A), (2) fixing Day/Night wind ratio (URATIO) to 2.0(B-l) and computing daily URATIO (B-2), and was compared with Actual Evapotranspiration (AET) for paddy fields in Suwon (1985-1986). The followings are a summary of this study results ; 1. Using 1985-1986 meteorological data, daily average PET in cases of A, B-i, B-2 were 4.61 mm/day, 4.81 mm/day and 5.36 mm/day respectively and daily average AET was 4.26 mm/day. The increment ratios of PET based on case A were 100%, 104.34% and 116.27% 2. The range of Adjustment factor (C) in cases of B-i, B-2 were 0.916-1.140 and 0.922-1.392 respectively. 3. The regression coefficient(r) between AET and PET in cases of A, B-i, B-2 were 0.928, 0.924 and 0.915 respectively.