An all-in-one computer includes a front enclosure and a rear enclosure fixed together. The front enclosure includes a rear piece opposite to the front enclosure, and two side pieces extending forwards from left and right ends, respectively. The rear piece defines an air outlet including a number of first holes each having a width or diameter between 1 mm and 2 mm. Each side piece defines an air intake including a number of vertically spaced elongated second holes. The front enclosure and the rear enclosure cooperatively bound an airtight space except communicating with outside via the air intakes and the air outlet. Each second hole is slanted lengthwise down towards the front enclosure with a first slanting angle of not less than 15 degrees. A bottom wall of each second hole is slanted down outwards with a second slanting angle of not less than 30 degrees.