This article presents a finite element scheme with Newton's method for solving the time-fractional nonlinear diffusion equation. For time discretization, we use the fractional Crank-Nicolson scheme based on backward Euler convolution quadrature. We discuss the existence-uniqueness results for the fully discrete problem. A new discrete fractional Gronwall type inequality for the backward Euler convolution quadrature is established. A priori error estimate for the fully discrete problem in L 2 (Ω) norm is derived. Numerical results based on finite element scheme are provided to validate theoretical estimates on time-fractional nonlinear Fisher equation and Huxley equation. KEYWORDS discrete fractional Gronwall type inequality, error estimates, fractional Crank-Nicolson method, time-fractional diffusion equation 1 Present address Sudhakar Chaudhary,