Voltage instability is a phenomenon related to very loaded systems associated, in general, with the demand of reactive power. Voltage collapse involves all or almost all the system. For this reason, is very important to consider this phenomenon and its prevention in order to improve the power system operation. There are several methods used to evaluate voltage stability margins, some of which give structural information of the behavior of the system around its voltage stability. Other kinds of methods are based on bus indexes giving local information about the voltage stability of the system. This paper is focused on including the L-index as a local constraint of voltage stability margin, to be used in the computation of the Optimal Power Flow (OPF). The program is implemented in Matlab using the PSAT-GAMS interface. The methodology is tested using a 6 bus system and the IEEE-RTS 24 bus system.