Let G=GL(m|n) be a general linear supergroup over an algebraically closed field k of odd characteristic p. In this talk we establish Jantzen filtration of Weyl modules V(λ) for G when λ is a typical weight in the sense of Kac's definition, and consequently obtain a sum formula for their characters. By Steinberg's tensor product theorem, it is enough for us to study typical weights with aim to formulate irreducible characters. As an application, it turns out that an irreducible G-module L(λ) can be realized as a Kac module if and only if λ is p-typical. This is a joint work with Yiyang Li.