A memory allocation method for a direct memory access controller (DMAC) in a limited-memory-size computer system includes the steps of allocating a memory space having continuous memory addresses to form a buffer of the DMAC; dividing the memory space successively into a plurality of first memory blocks and a second memory block, wherein the size of the second memory block is equal to a maximum frame size possibly accessed by the DMAC; and assigning the plurality of first memory blocks and the second memory block to a plurality of descriptors in order, wherein each of the plurality of descriptors is utilized for recording a memory address of a corresponding memory block as a pointers for the corresponding memory block.