A survey was carried out in order to figure out of phenomenon of increasing and decreasing rate of WBC during a day in normal and healthy person from Oct. 1, 1972 to Nov. 31, 1972. For this survey, it was classified by two groups 50 male students and 35 female students who are from 18 to 25 years old. The blood sampling was collected by thin smear method 8 times from 7:30 A.M. to 10:30 P.M. by 2 hours intervals and calculated by direct manual method. As the results of this survey, the following conclusions were obtained 1) Number of WBC was the lowest degree during the forenoon, and it was the highest degree around the midnight. It was increase from around the noon up to the midnight and decrease from the midnight up to around the noon. 2) Increasing and decreasing curve of WBC of male and female was keeping same pace with both sex in a day. 3) Increase and decrease of lymphocyte and eosinophil was keeping almost same pace with WBC. 4) Increasing rate of number of monocyte was more speedily increasing during the afternoon than the night.