The paper presented a newly conceived vocabulary size test based on pictorial cues: Pictorial Vocabulary Size Test (PVST). A model-based (1-2-3 parameter logistic item response theory model comparisons) approach was taken to check which model could absorb the most information from the data. Junior high school and primary school students participated in the study (N 1,354). Subjects’ ability estimates and item parameter estimates were computed based on expected a posteriori (EAP) method, one type of Bayesian method. BILOG-MG 3 was adopted to execute parameter estimates and model comparisons. The results showed that the 3PL-IRT model best fit the empirical data. It was then argued that test takers’ English vocabulary size could be best captured under the 3PL-IRT model, as not only the discrimination parameter, but also the guessing parameter has a fundamental role to play in consideration of the test format adopted in the PVST. The article concluded that the PVST could have positive washback effects on test development and English vocabulary instruction.