This paper presents the regression equation for predicting compressive strength of in-situ concrete. For this purpose, this study used the data of mixture proportions of ready-mixed concrete and test results of compressive strength at construction sites.This study used 1442 compressive strength test results obtained from the specimens having 59 different kinds of mixtures with specified compressive strength of 18~27MPa, water-cement ratio of 0.39~0.62, maximum aggregate size of 25mm, and slump of 12~15cm.Principal factors that influence compressive strength of concrete are selected by a correlation analysis, and then the multiple linear regression analysis is carried out for predicting compressive strength according to water-cement ratio or cement-water ratio, cement contents and cement-aggregate ratio.