An evaporator suitable for absorbing heat from a heat source is provided. The evaporator includes a top board, a bottom board, a side frame, and at least one porous member. The side frame connects the top board and the bottom board. The porous member is disposed between the top board and the bottom board and within the side frame. The part of the top board covering the porous member is a heat conducting portion near the heat source. The evaporator has at least one first channel, at least one second channel, a fluid inlet, and a fluid outlet. The first channel is adjacent to the bottom board and the porous member for containing a working fluid. The second channel is adjacent to the top board and the porous member for containing the working fluid. The fluid inlet communicates with the first channel. The fluid outlet communicates with the second channel.