We investigated the effect of thulium ion concentration on the continuous-wave (CW) power performance of diode single-end-pumped thulium-doped YAlO3 (Tm:YAP) lasers. Three samples with 1.5%, 3%, and 4% Tm3+ concentration were examined at 18°C. Lifetime and fluorescence measurements were further performed to assess the strength of cross relaxation and nonradiative decay. Our results showed that in single-end-pumped configurations, the best CW power performance was obtained with the 1.5% Tm:YAP sample, and laser performance of the samples degraded monotonically with increasing Tm3+ concentration. By using 9.5 W of incident pump power at 797 nm, a maximum of 1430 mW of output power was obtained with the 1.5% Tm:YAP sample and 2% output coupler. We discuss how the effects of cross relaxation, reabsorption, nonradiative decay, and internal heating vary with increasing concentration. Spectroscopic measurements and rate-equation analysis suggest that cross relaxation should already be effective in samples with 1.5% Tm3+ ion concentration and doping concentrations larger than 4% will lead to degradation in power performance due to higher nonradiative decay rates and larger reabsorption losses.