In this paper, a preventive maintenance warranty (PMW) policy for repairable products is investigated. The objective is to determine the optimal number of preventive maintenance (PM) actions, the degree of maintenance and the length of each PM interval; while minimising the total expected warranty cost. The failure rate of repairable products is assumed to be periodically increasing over time. A unique relationship between successive PM intervals is determined. For a special case of failure rate and minimal repair cost, a relationship among the PM intervals is derived. A unique property is inferred from this special case, which relates the maximum failure level in the adjoining PM intervals. The derived relations are illustrated by a numerical example. Yeh and Lo presented a mathematical model for the optimal determination of a PMW policy under similar periodic failure rates. The present study is an extension of their work and shows that unequal PM intervals provide an economic benefit.