In this paper we examine the average Rényi entropy S α of a subsystem A when the whole composite system AB is a random pure state. We assume that the Hilbert space dimensions of A and AB are m and mn respectively. First, we compute the average Rényi entropy analytically for m = α = 2. We compare this analytical result with the approximate average Rényi entropy, which is shown to be very close. For general case we compute the average of the approximate Rényi entropy, which is in agreement with the asymptotic expression of the average von Neumann entropy. Based on the analytic result of S α (m, n) we plot the ln m-dependence of the quantum information derived from S α (m, n). It is remarkable to note that the nearly vanishing region of the information becomes shorten with increasing α, and eventually disappears in the limit of α → ∞. The physical implication of the result is briefly discussed.