Abstract: In this paper，the definition of theoretical relative deviation from Γ-information is introduced in two-valued propositional logic.Meanwhile the conditional truth degree expressions are showed and a new conclusion mode that the formula derived from theoretical relative deviation is the 3-typical error of less than ε in the abstract is put forward.At the same time the equivalence relations are proved among the conclusion modes for the 1，2-typical errors of less thanε.These studies can diversify research means of the approximate reasoning problems in two-valued propositional logic system.