In this paper we consider a class of polynomial planar system with two small parameters,ε and λ,satisfying 0 < ε << λ << 1.The corresponding first order Melnikov function M1 with respect to ε depends on λ so that it has an expansion of the form M1 (h,λ) =∞∑k=0 M1k(h)λk.Assume that M1k'(h) is the first non-zero coefficient in the expansion.Then by estimating the number of zeros of M1k'(h),we give a lower bound of the maximal number of limit cycles emerging from the period annulus of the unperturbed system for 0 < ε << λ << 1,when k'=0 or 1.In addition,for each k ∈ N,an upper bound of the maximal number of zeros of M1k(h),taking into account their multiplicities,is presented.