A heat dissipation unit includes a main body having a first and a second plate member, which are closed to each other to together define an airtight chamber in between them. A working fluid is filled in the airtight chamber, and a first wick structure layer and a holding-down thin layer are provided between the first and the second plate member and superimposed in the airtight chamber. With the holding-down thin layer and the first wick structure layer being superimposed in the airtight chamber, the first wick structure layer is closely and flatly attached to the second plate member without warping.