An LED illuminating device includes an optical section, an electrical section, and a heat dissipation section provided with a heat dissipation device. The heat dissipation device includes a hollow metal tube, a bottom cover plate and a top cover plate respectively sealing a bottom and a top of the metal tube, and a tubular member received in the metal tube and axially defining a vapor passage channel therethrough. A condensate passage channel is defined between the metal tube and the tubular member. An evaporating chamber is defined between the bottom of the tubular member and the bottom cover plate. A condensing chamber is defined between a top of the tubular member and the top cover plate. First and second porous wicks are in the metal tube and attached to the bottom cover plate and the condensate passage channel respectively. A working fluid is saturated in the first porous wick.