The purpose of this study is to investigate on the level of the underesting of probability concept. The result were shown as follows: 1. Mixing and irreversibility, quantification of probability, permutation, combination and arrangement were apposite to age level but symmetric and uniform distribution, balloting by pairs were late to age level of probability concepts. 2. The difference between boy and girl appeared in 6 grade well on formation of probility concept but it is appear little or not in 4 grade and 5 grade.