The cucumbers were grown in three kinds of nutrient 150 ppm NO3^- -N, 150 ppm NH4^+ -N, and the mixture of 75 ppm NO3^- -N and 75 ppm NH4^+ -N, separately for seven days. The best and the worst growth were in the nutrient of NO3^- -N and NH4^+ -N, respectively. Although they took the same amounts of combined nitrogen from each nutrient, the better assimilation of NO3^- -N than NH4^+ -N in the cucumbers resulted in heavier dry weights, higher contents of total nitrogen, and higher soluble proteins contents in the leaves of cucumbers that grown in higher concentration of NO3^- -N. The compositions of soluble proteins in the leaves of cucumbers were also influenced by nitrogen sources by showing their discrete electrophoretic patterns.