This experiment was undertaken in order to clarify effect of NO$_3$-N/NH$_4$-N ratios(NO$_3$/NH$_4$ : 10:0, 8:2) in the nutrient solution on growth, yield, photosynthetic rate, relative concentration of chlorophyll and root activity of hydroponically grown pepper plants at three different root- zone temperatures of 18$^{circ}C$, 22$^{circ}C$ and 26$^{circ}C$. Plant height, leaf number, stem diameter, fresh and dry weight of leaf and root were no effect in by three root- zone temperatures. However, leaf number, stem diameter, fresh and dry weight of leaf and stem, dry weight of root at 18$^{circ}C$, 22$^{circ}C$ and $25^{circ}C$ increased when NH$_4$-N was added to the solution. Under root-Bone temperatures of 18$^{circ}C$, 26$^{circ}C$ condition, fruit length were longer by the addition of NH$_4$-N. Fruit number and yield increased by the addition of NH$_4$-N at three root-zone temperatures. Photosynthetic rate decreased as root - zone temperature increased. Under root-zone temperatures of 18$^{circ}C$, 22$^{circ}C$ and 26$^{circ}C$ condition, photosynthetic rate increased significantly by the addition of NH$_4$-N. Chlorophyll content of plants increased at 22$^{circ}C$. Under root-zone temperatures of 18$^{circ}C$, 22$^{circ}C$ and 26$^{circ}C$ condition, chlorophyll content of plants increased by the addition of NH$_4$-N. Root activity of increased at 26$^{circ}C$ Under root-Bone temperatures of 18$^{circ}C$, 22$^{circ}C$ and 26$^{circ}C$ condition, root activity increased by the addition of NH$_4$- N.