Abstract: In order to connect evaporation data observed from 20 cm diameter evaporation pans in the past 40 years with those from E-601B pans, a comparison of evaporation data is made between 20 cm diameter pan and E-601B pan from 618 evaporation experimental stations for three years. The monthly and annual-mean conversion coefficients for the two instruments and the nationwide distribution chart of the annual-mean coefficient, as well as the corresponding standard deviation and relative standard deviation, are given.