By using fixed point theorem on the cone, the existence of positive solutions of fourthorder threepoint boundary value problem u(4)t=f(t,u(t)),t∈\[0,1\],u′(0)=u″(η)=u(0)=u(1)=0 is studied, where f:\[0,1\]×\[0,+∞)→\[0,+∞) is continuous,，η∈33,1 is a constant.Note that although the Green′s function is changing sign,the solution which is obtained is still positive.

Green′s function; boundary problem;fixed point theorem; positive solution