A portable computer includes a shell, a printed circuit board in the shell, a heat generating element, a heat conducting sheet, a primary heat dissipation unit, a subsidiary heat dissipation unit, and a heat-transfer unit. The heat generating element is electronically connected on the printed circuit board. The heat conducting sheet is positioned on the heat generating element. The heat-transfer unit includes a heat-transfer member and a drive unit. The heat-transfer member connects to the heat conducting sheet. The drive unit moves the heat-transfer member to connect with the primary heat dissipation unit or the subsidiary heat dissipation unit.