In the absence of a polynomial time algorithm capable of obtaining the exact solutions to it, the domatic number problem (DNP) of dominating set (DS) has been regarded as NP-complete. This paper suggests polynomial-time complexity algorithm about DNP. In this paper, I select a vertex of the maximum degree as an element of a dominating set ⋯ , compute from a simplified graph of ╲ , and verify that is indeed a dominating set through ╲ . When applied to 15 various graphs, the proposed algorithm has succeeded in bringing about exact solutions with polynomial-time complexity . Therefore, the proposed domatic number algorithm shows that the domatic number problem is in fact a P-problem.