Abstract: In this study, an order-level inventory model is constructed for deteriorating items with instantaneous replenishment, exponential decay rate and a time varying linear demand without shortages under per-missible delay in payments. It is assumed that a constant fraction of the on-hand inventory deteriorates per unit of time. The exact formulae of the optimal average cost and the lot size are derived without carrying out any approximation over the deterioration rate. Different decision making situations illustrated with the help of numerical examples. Sensitivity analysis of the optimal solution with re-spect to changes in the parameter values is carried out.