In this paper, it is defined that the failure criterion of coated abrasive belts under constant force plunge grinding is the time when the stock removal rate is reduced to zero and under fixed feed plunge grinding is the time when the normal grinding force reaches to the specified value. Then the tool-life equation is derived theoretically on the basis of the metal removal mechanism and wear process of coated abrasive belts which have been reported in the previous papers. This tool-life equation may be considered to be the one which introduces the initial wear into the Taylor equation. And then stock removal during a belt life, the machining cost and the time to change belts are theoretically and experimentally investigated. As the result, optimum conditions of belt grinding on the total stock removal and the machining cost are reasonably determined.