Y1.8-xYb0.2HoxMo4O15 (x = 0.005, 0.01, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64) phosphors were synthesised at 700 degrees C by using a solid-state reaction method. The samples were characterised by X-ray diffraction, diffuse reflectance spectra, photoluminescence and up-conversion emission spectra. The photoluminescence spectra of Y2Mo4O15:Yb, Ho shows extremely strong green down-shift luminescence corresponding to the F-5(4), S-5(2) -> I-5(8) transition of Ho3+ ions with I-g/I-r > 20 under 454 nm excitation. While at 980 nm excitation, Y2Mo4O15:Yb, Ho sample exhibits unusual red up-conversion luminescence along with a very weak green emission (I-r/I-g = 6.4), which is resulted from the severe non-radiative relaxation (I-5(6) -> I-5(7)) process due to the very short decay time of I-5(6) states of Ho3+ ions. The mechanism of down-shift luminescence is the electrical dipole - dipole transitions. The quenching concentration for down-shift luminescence is higher than that for up-conversion luminescence, which results from the effects of different luminescence processes and large distance between doped rare earth ions in Y2Mo4IO15. The different luminescence mechanism and color performance are also studied.