We propose a new double-block-length compression function which is called Alpha-DBL. This scheme using two parallel secure single block length schemes based on a block cipher with 2n-bits key and n-bits block size to compress a 3n-bits string to a 2n-bits one. We show that Alpha-DBL scheme attains nearly optimal collision security and preimage security bounds (up to 2ⁿ and 2²ⁿ queries for finding a collision and a preimage, respectively). More precisely, for n = 128 no adversary making less than 2^n-1.27 = 2^126.73 queries can find a collision with probability greater than 1/2. With our knowledge, this collision security bound is better than other of such a compression function. In addition, we give a preimage security analysis of Alpha-DBL that show security bound of 2^2n-5 = 2²⁵¹ queries for n = 128. Using this scheme in the iterated hash function construction can preserve the collision resistance security and the preimage resistance security.