Cubic KLu3F10 nanocrystals have been fabricated by doping Mn(2+) ions via a liquid-solid solution (LSS) hydrothermal method. With an increase in the Mn(2+) level, a phase transformation from orthorhombic KLu2F7 to cubic KLu3F10 associated with evolution of the particle morphologies and sizes is evident. The phase transformation mechanism based on doping with Mn(2+) ions was studied via a series of time-controlled experiments and the enhancement of the intensity of upconversion luminescence (UCL) via doping with Mn(2+) ions is discussed in detail. In addition, under 980 nm excitation, the UCL properties of Er(3+)/Tm(3+)/Ho(3+)-doped KLu3F10:Yb(3+) upconversion nanoparticles (UCNPs) have been investigated and a possible UCL mechanism has been discussed. It has been demonstrated that cubic KLu3F10 is an excellent host for upconversion luminescence.