A heat dissipation device includes a heat sink, a fan adapter and a fan. The fan adapter has two opposing sidewalls straight arranged thereof, a top wall disposed horizontally and connecting the two sidewalls, an inlet formed in a front thereof, an oblique reduced cross-sectional area portion extending backwardly from a rear thereof, and an outlet formed in a rear of the oblique reduced cross-sectional area portion. The fan adapter encloses the heat sink; the fan disposes on the inlet of the fan adapter. Whereby heat around electrical components, on which the heat dissipation device is arranged, can be carried out by a fluid flows passing through the oblique reduced cross-sectional area portion.