The photofraction, i.e., the fraction of incident photons absorbed, and the Compton electron distribution of a very large NaI(Tl) scintillator are investigated. They are estimated from the measurement of energy spectrum of photons backscattered from the front face of a scintillator that is nearly semi-infinite for 60Co and 137Cs gamma rays. The estimated photofraction was 0.79 for 60Co gamma rays and 0.85 for 137Cs gamma rays. The Compton electron distribution obtained was compared with the results of Monte Carlo calculations. They show fairly good agreement.