Increase of the conversion efficiency of radiation at 13-14nm is the most important issue for making EUVL affordable. An analytical formula for maximizing the radiation efficiency from a laser-produced plasma has been derived. The maximum efficiency is achieved when the plasma expansion distance during laser heating is equal to the laser absorption length. Theoretically predicted dependence of the radiation efficiency on the plasma density was confirmed in a experiment using a particle-cluster target. By creating a relatively uniform density plasma with a 0.3 mm diameter by dispersing SnO₂ particles coated on a Si wafer, the conversion efficiency at 14-nm as high as 4 times of that for a solid density SnO₂ target was observed.