A latch device for heat dissipation unit is applied to and mounted on a motherboard in tight contact with at least one heat source. The latch device includes a heat sink, a first elastic abutment member having a first protrusion block and a second elastic abutment member having a second protrusion block. A first latch member and a second latch member are disposed on two opposite sides of the heat sink. The first and second latch members are freely latchable with the corresponding first and second protrusion blocks or unlatchable from the corresponding first and second protrusion blocks. Accordingly, along with the latching or unlatching, the first and second elastic abutment members are elastically deformed into abutment against or out of abutment with the other face of the heat sink. Therefore, the heat sink can be easily and securely fixed on or detached from the motherboard.