Elephant random walk is a kind of one-dimensional discretetime random walk with infinite memory: For each step, with probability α the walker adopts one of his/her previous steps uniformly chosen at random, and otherwise he/she performs like a simple random walk (possibly with bias). It admits phase transition from diffusive to superdiffusive behavior at the critical value αc = 1/2. For α ∈ (αc, 1), there is a scaling factor an of order n α such that the position Sn of the walker at time n scaled by an converges to a nondegenerate random variable W , whose distribution is not Gaussian. Our main result shows that the fluctuation of Sn around W · an is still Gaussian. We also give a description of phase transition induced by bias decaying polynomially in time.