This study explored the relationship between successful guessing and latent ability in dichotomous IRT models. Two new IRT models, the Rasch-Guessing model and the 2PL-Guessing model were developed with guessing functions integrating probability of guessing an item correctly with the examinee’s ability and the item parameters. The conventional 3PL IRT model was compared with the new 2PL-Guessing model on parameter estimation using the Monte Carlo method. SAS program was used to implement the data simulation and the maximum likelihood estimation.