An illumination system adapted for a projection apparatus is provided. The illumination system comprises a cooling air guiding apparatus, a lamp, and a cooling fan. The cooling air guiding apparatus comprises an air intake device and an adjustment device, while the adjustment device comprises an air-flow adjustment element and an air-flow guiding element. The cooling fan generates a cooling air, which flows from the exterior through the air intake device into the air-flow adjustment element. The air-flow guiding element guides the cooling air in the air-flow adjustment element out of an air-flow adjustment hole towards a specific direction. Since the adjustment device may move relatively to the air intake device, the air-flow adjustment hole is adapted to move from the first position substantially aligned with the opening to the second position constituting a substantial displacement with respect to the opening. The cooling air can cool the lamp no matter what angle the projection apparatus is disposed. As a result, the projection apparatus is allowed to be installed in any desired orientation without restrictions.