The existence of positive solutions are obtained for boundary value problems of impulsive differential equation
-y″(t)=f(t,y(t)), t∈［0,1］＼｛t_{1},t_{2},…,t_{m}｝,
Δy′(tk)=J_{k}(y(t_{k})), k=1,…,m,
y(0)=y(1)=0
where f can change the sign. The proof is based on the fixed point index theorem in cones. The results of this paper improve and generalize the known results in the literature.