In this work, a model parameter extractor of solar cells using Matlab is shown. The software, developed in Matlab environment, has been successfully applied to study internal characteristics of photovoltaic devices [1-2], as well as on-line photovoltaic system performance [3-4]. However, in this case, the developed tool allows to obtain automatically electrical parameters than are often requested to model the performance of single solar cells, as can be the photocurrent density Jph, series resistance Rs, shunt resistance Rsh, saturation current Io, and ideality factor n. Parameters of a second diode, that takes into account recombination losses, as can be the saturation current Iorec and ideality factor nrec, are also included . Special attention has been focused on the extimation of the series resistance Rs, and the evolution of this parameter in dark and illumination work conditions of the solar cell.