The effect of relative humidity (RH) and temperature on the microbial behavior of Escherichia coli O157:H7 on lettuce was investigated. Experimental data obtained under different combined conditions of RH (60, 70, and 80%) and temperature (15, 25, and $35^{circ}C$) were fitted into the Logistic model with delay to estimate specific growth rate (SGR) with high coefficients of determination ($R^2$ >0.98). The estimated SGR values were used to develop a Gamma-concept model. Then, several statistic characteristics were employed to verify the performance and reliability of the developed model, which demonstrated that the predictive model was not biased and had high accuracy in prediction. Besides, the analysis of relative importance of temperature and RH indicated that temperature is much more influential on the growth of E. coli O157:H7 on lettuce than the ambient RH changes.