Because of its peculiar mechanism for plasminogen activation and the molecular evolutionary relationship with serine protease, streptokinase had long been investigated by many researchers. But, there are little reports on the structure of streptokinase. So, to elucidate the structural and functional roles of C-terminal amino acids of streptokinase, various C-terminal deletion mutants were constructed by nested deletion of streptokinase gene with exonuclease III. After casein/human plasminogen overlay and CLN hydrolysis test, it was observed that 40 residue deletion from C-terminus did not affect the plasminogen activation efficiency. But, deletion of 46 or more amino acids had dramatically reduced the plasminogen activation efficiency. So, residue 40 to 46 from C-terminus did appear to have dramatic effect on the efficiency of plasminogen activation. It is not clear at present why the loss of 46 or more amino acids resulted in the reduction of plasminogen-activation efficiency. However, from the facts that $\beta$ -turn structure generally existed in protein surface and could be interacting site with other protein, it could be proposed that the predicted $\beta$ -turn structure between residue 369 to 372 was engaged in binding to plasminogen by electrostatic interaction.