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.