The multileaf collimator (MLC) is becoming a standard accessory of modern linac in shaping radiation fields. However, for a given target (projection), the radiation field shaped by an MLC has a stepwise boundary and is not identical to the desired field that exactly conforms to the target. That means there are always under-blocked and/or over-blocked areas. The total area of discrepancy depends on MLC leaf widths. The purpose of this study is to develop an optimization model for determining leaf widths so that the total area of discrepancy between MLC-shaped fields and the desired ones can be minimized. The optimization model regards leaf widths as variables, the total area of discrepancy between MLC-shaped fields and the desired fields as an objective function, and the total width of all leaves as a constraint. A problem described by the model is solved with the hybrid of a simulated annealing technique (ASA, Lester Ingber, 1993) and a gradient technique (DONLP2, P Spellucci, 2001). The performance of the optimization model was evaluated on 634 target fields continuously selected from the patient database of a treatment planning system. The lengths of these fields ranged from 3.9 to 38.7 cm and had an average of 15.3 cm. The total area of discrepancy was compared between an MLC with optimal leaf widths and a conventional MLC with the same number of leaf pairs. Optimal leaf widths were obtained for an MLC with total leaf pairs of 28, 40 and 60, respectively, which corresponded to three types of conventional MLCs. The optimal leaf width first decreases slightly and then nonlinearly increases with the distance away from the central line. Compared with the MLC with conventional leaf width arrangement, the MLCs with optimal leaf width arrangement reduced the total area of discrepancy by 11.1%, 28.6% and 25.0%, respectively. Optimizing leaf widths can either improve the conformity of MLC-shaped fields to the treatment targets when the number of leaf pairs does not change, or reduce the number of leaf pairs without sacrifice of field conformity.