Abstract: It is given a simple proof of Cramer Rule in Linear Algebra textbooks. As we all know, system of linear equations can be solved by means of Cramer Rule when its coefficient matrix is a square matrix and it has a single solution. But it is powerless for Cramer Rule to solve when its coefficient matrix isn’t a square matrix. So, it is promoted Cramer Rule to solve the arbitrary system of linear equations as long as it has a single solution.

Key words: system of linear equations, single solution, cramer rule, rank