A new class of perfect nonlinear binomials was just found by Helleseth et al, which are the first perfect nonlinear binomials composed with two inequivalent monomials. We transformed the class of perfect nonlinear binomials to another form, and gave a concise proof for their perfect nonlinearness. It shows that this family of binomials is equivalent to . Furthermore, the calculation of the count of this family of functions is presented as well.