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Using the rate equations describing the dynamic processes of the system, how the strength of the upconversion luminescence change with the Er3+ ion concentration and the pump power was analyzed. The spontaneous radiative processes, multiphonon nonradiative processes, absorption of the pump photons and the possible energy transfer processes between all states of Er3+ ions were included in the rate equations, and the pump power was supposed to be unchanged with time. It came to the result that when x change from 0.1% to 100% in ErxLa1-xP5O14 crystal, lager x benefit the population of all states higher than 4F9/2 in the whole range of pump power we considered, except for a small range of pump power in which smaller concentration result in larger ion number of some high lying states. It can be interpreted as that the increasing of the concentration of Er3+ ions increases both energy transfer processes which populate the upper states of the upconversion luminescence and the cross relaxation processes which depopulate these states. Since the strength of upconversion luminescence is in proportion to the ion numbers of the upper states of the luminescence, the total result is that the upconversion luminescence from 4S3/2 would increase with the concentration of Er3+ ions, and the strength of this upconversion luminescence were most strong in ErP5O14 crystal; for a small range of pump power, the strength of upconversion luminescence from level (2G4F2H)9/2 and 4G11/12 of Er ions in Er0.8La(subscript 0.2P5O14 crystal was most strong, for all the other range of pump power, this luminescence was also strongest in ErP5O14 crystal.
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