Improvement of the efficiency of a Rh6G dye laser by adding proper concentrations of gold colloidal nanoparticles into the dye solution has experimentally been investigated. Gold colloidal nanoparticles due to their strong plasmonic peak in the visible region are expected to have a considerable interaction with Rh6G dye molecules with broad emission spectrum in the visible range. It is shown that with addition of enough low concentrations (less than 0.4 µM/l) of gold nanoparticles to the dye solution, both the output power and the efficiency of the dye laser increase. The best improvement of the laser efficiency reaches 15%. Two amplifying mechanisms exist for efficiency improvement by the nanoparticles, which are resonant energy transfer from the plasmonic nanoparticles to the dye molecules in addition to the high near field around the nanoparticles. The results indicate that in the low concentrations of gold nanoparticles, the amplifying mechanisms dominate the dissipating mechanism of scattering of the pumped light by the nanoparticles. This leads to the efficiency improvement by addition of the nanoparticles.