Among various biological effects of extremely low frequency (ELF) electric field, we have found the increase in peripheral blood flow when the field is exposed to human body surface. To elucidate this phenomenon, we investigated the change of red blood cell (RBC) aggregability. Volunteer subjects stopped taking food at least 10 hours including sleeping period before the experiment. Blood was sampled according to the predetermined schedule, and put on a slide glass for microscopic observation. The microscope images of RBCs were analyzed with an image-processing software. It classified the RBCs into several groups such as; normal, aggregated, megacytic, microcytic and degenerated RBCs. In this study, we watched the ratios of the normal and aggregated RBCs as a relevant parameter to the blood flow change. The subject was exposed to the electric field using a therapeutic electric field exposure instrument (Healthtron, HES-30). It applied high voltage (30 kV) beneath the subject's sole of the feet. The subject body was electrically floated from the ground with the insulating chair. High electric field (20-250 kV/m) was generated between the subject body and the grounded environment.