The influence of respiration on photosythetic electron transport were investigated in the Wid type and psaB mutants from Syneehocystis sp. PCC6803. The amount of glucose uptake in the wild type was proportional to the glucose concentration added in wild type and less than that of psaB mutants in the dark. It was suggested that psaB mutants more depend on the glucose than the wild type. It was investigated how the activities of isocitrate dehydrogenase(IDH) and glucose-6-phos-phate dehydrogenase(G6PDH) were changed. The activities of IDH were very low. While, the ac-tivities of G6PDH were much higher than that of IDH. These results agree to the reports that ex-ogenous glucose was dismilated aerobically via Oxidative Pentose Phosphate Pathway in heterotrophic cyanobacteria. PsaB mutants showed high G6PDH activity in the presence of glucose as well as in the dark and high respiratory activities especially in the dark. It was also investigated how photosynthetic electron transport activities were changed. PsaB mutants showed higher photosynthetic electron tranasport activities than wild type in the presence of glucose as well as in the dark. In the results, it was proposed that photosynthetic electron transport between PS I and PS U was complemented by respiratory electron transport through the NADPH generated by Dxidative Pentose Phophate Pathway in psaB mutant from Synechocystis sp. PCC6803. Key words: Photosynthetic electron transport, Respiration, Synechoystis sp. PCC6803, psaB mutant, Glucose uptake, IDH, G6PDH, Respiratory electron transport activity.