Reducibility conjecture is one of old Question in 3-manifold thoery. The conjecture is that if both M(r1) and M(r2) are reducible, then intersection number between r1 and r2 is less than 2. C.M.Gordon and J. Luecke proved this conjecture. In this talk, we observe the thechnique of Ying-Qing Wu and discuss the results.