Bayesian theory is applied for calibration of Hargreaves model for estimating daily ET0 in Sichuan basin of southwest China. The calibrated Hargreaves models performed much better than the original one for estimating ET0. Both the original and calibrated HG models overestimated ET0 at daily, monthly and annual timescale. The calibrated HG model could be highly recommended for estimating ET0 when only temperature data are available.