A common method for studying problems about the length of service and grade seniority by Markov manpower model is to divide the states of the model according to grades，lengths of service or grade seniority. In a class of graded manpower systems using homogeneous Markov models in which there are no demotions and in which promotions take place only step by step，the present paper discussed the problems of lengths of service and grade seniority，some computation formulae describing the lengths of service and grade seniority intuitively have been obtained，and finally，a practical example demonstrating the applications of this method is given.