  Let $M_{z}$ be the multiplication operator on the Bergman space and $M_{I}$
denote the restriction of $M_{z}$ to an invariant subspace $I$. A question
raised by K. Zhu is that when are two restriction operators $M_{I}$ and $M_{J}$
are similar? In this note, we give some sufficient conditions of this problem
in a general case.
