Abstract: Photoreduction activities of isolated chloroplasts of hybrid rice cv.Shangyou 63 could be effectively increased by additional Ca²⁺,and inhibited by ethyleneglycol-bis(β-aminoethyl ether)-N, N, N, N-tetraacetic acid (EGTA, a specific Ca²⁺ chelator ).Trifluoperazine (TFP,a calmodulin inhibitor), Co²⁺ (a calcium channel blocker),and Zn²⁺ inhibited photoreduction activities of isolated chloroplasts,and which were partially decreased by the Ca²⁺.Photoreduction activity of chloroplasts was close related to Ca²⁺ andcalmodulin.