Abstract: A mathematical model of dew duration on cucumber leaves in film plastic greenhouse was developed useing the theory of energe balanceon single leaf. The difference between the calculated value and measured of The leaf temperature, dew duration and the time of beginning and drying to dew was not obvious. The average values were 21, -6 and -26 minutes separately. It was obtained, depend on the relation between the frequency of leaf wet and ralative humidity, that the critical value of relative humidity of beginning to dew on cucumber lesves in film plastic greenhouse was 93% The calculating error was less than one houre.