In 2000, Walter Wyss looked into the fractional version of the Black-Scholes equation for the first time. He gave a solution of the fractional Black-Scholes equation by using the Greens function [14]. In this paper, Lie symmetry analysis of a time fractional Black-Scholes equation with Riemann-Liouville derivative is performed. The operators admitted are obtained and finally an example of the invariant solution of the equation is discussed.