In 1750, the Swiss mathematician Gabriel Cramer published a well-written algebra book entitled "Introduction a l'Analyse des Lignes Courbes Algebriques." In the appendix to this book, Cramer gave, without proof, the rule named after him for solving a linear system of equations using determinants (Kosinki, 2001). Since then several derivations of this rule have appeared (Chadha, 1996; Larson et al., 2004; Shirm & Adams, 2002). In this article, the author will introduce another proof of Cramer's rule based on the expansion in cofactors of a determinant.