  We analyze multi-instanton sector in two dimensional U(N) Yang-Mills theory
on a sphere. We obtain a contour intregrals representation of the
multi-instanton amplitude and find ``neutral'' configurations of the even
number instantons are dominant in the large N limit. Using this representation,
we calculate 1,2,3,4 bodies interactions and the free energies for $N =3,4,5$
numerically and find that in fact the multi-instanton interaction effect
essentially contribute to the large N phase transition discovered by Douglas
and Kazakov.
