I read a paper "Notes On Stable Maps And Quantum Cohomology, W.Fulton and R.Pandharipande". And I think that $\overline{M_{0,n}}(\mathbb{P}^n,d)$ is irreducible. But I cannot find an exact statement from any references.
It will be very grateful to tell me about reference about this, or whether some restrictions to $n,d$ is needed for the irreduciblility.