Diurnal changes in the gene expressions and activities of Rubisco and Rubisco activase (RCA) in different position of leaves of cucumber were investigated. The results showed the diurnal variations of the mRNA abundance of rbcL, activities of Rubisco and RCA in 4th apical leaf which gave a single-peak curve, which was similar to the Pn curve on a sunny day. However, the mRNA abundance of rbcS and CsRCA showed a double-peak pattern in diurnal changes. The mRNA abundance of rbcL and the activities of Rubisco and RCA were the highest in the 4th and 8th apical leaves, lower in 12th, and the lowest in the 1st leaves. The mRNA abundance of rbcS in 4th to 8th leaves was also found to be the highest, but that in 12th leaves was lower than that in 1st leaves. The mRNA abundance of CsRCA increased successively as leaf position declined. Different position of leaves showed the same changing patterns in Pn as compared to the activity patterns of Rubisco and RCA. These data indicated that the mRNA expression of rbcL and the activities of Rubisco and RCA in cucumber leaves are involved in diurnal changes of net photosynthetic rate. Key words: Photosynthetic rate, gene expression, diurnal variation, leaf-position, Cucumis sativus L.