In this article we obtain for the arithmetic progression of positive integers ki-r an asymptotic formula, namely Pn i=1 (ki-r) ~ arknnnn-r/k+1/2/en (k>0) (r = 0, 1, . . ., k-1), which generalizes the Stirling’s formula for the sequence i of all positive integers. In this formula ar is a positive number which depend of r for a fixed k. We also obtain some information of the product Pn i=1 pi where pi is the i-th prime number.