The origin of collision operator in kinetic equation and its connection with the problem of increasing entropy are reviewd. In this review, it is checked that some kind of smoothing procedure is needed to get nontirvial collision operator. If we assume that the origin of increasing entropy is coarse-graining, it can be shown that the collision operator, which is obtained by smoothing introduced by coarse-graining, plays an important role in increasing entropy. So an iterative filtering for coarse-graining is introduced and some general relations for filtered quantity are derived. Using those relations, the perturbative form of collision operator is proposed and the dependency of coarse-graining size on collision operator is investigated. The present study shows that the collision operator has logarithmic dependency with coarse-graining size.
