The resonance radiation trapping in alkaline-earth metal atomic lasers were investigated according to Holstein's theory. The threshold temperature, ground-state density and resonant level effective lifetime of the resonance radiation trapping were calculated for Ca, Sr, and Ba atomic vapor lasers with different tube radii. The calculated threshold temperature is found to be in good agreement with the experimental laser-starting temperature, which indicates the resonance radiation trapping is a significant factor in establishing the population inversion of the resonance to metastable transition lasers.