Laboratory freezing and thawing tests were performed to undertake to provide comprehensive laboratory data on air entrainment which influence to frost resistance of normal weight aggregate concrete at early ages. And it is the aim of this study to present qualitatively and quantitatively a suitable air entrainment which is valid and necessary to prevent the early frost damage of normal weight aggregate concrete by comparing and investigating air content with the other factors influencing to frost damage of concrete at early ages. On the basis of these test conditions, the following observation and conclusions would appear to be warranted; 1. The use of intentionally entrained air results in an excellent effect in frost resistance of concrete at early ages. In case of normal weight aggregate concrete, the adequate amount of intentionally entrained air was found to be 4 percent or more of nominal air content in placing concrete, which is enough capable to get the inferential strength ratio of 80 percent or more after 5 cycles of freezing and thawing, and it is very clear that there is a correlation between air content and early frost resistance of concrete. 2. The influence of freezing temperature, freezing and thawing cycles and kinds of aggregate are not to be compared with intentionally entrained air in the effect of early frost resistance of normal weight aggregate concrete. 3. The strength and water cement ratio give comparatively a corresponding influence in frost resistance of concrete at early ages, while slump give little influence in frost resistance of concrete at early ages. 4. The restoration of strength after early frost damage is to be increased by the curing in water and humid air, but it is considered in actually impossible to expect in practical structures.