A new method

taking the molecules as circle unit

which is different from Flory and others' method

which took the functional groups as circle unit

has been used to derive the equation for the critical condition of gel formation of the reaction between ∑Aai and ∑Bb1 kinds of molecules. The new equation

expressed with number-average functionality or number fraction of molecules

instead of weight-average functionality or number fraction of functional groups

is more agreeable to Flory's experimental results. Based upon the theroretical analysis

the chemical reaction is molecular reaction. Therefore

it is more reasonable to use molecules as circle unit than to use functional groups as circle unit.