Abstract: The purpose of the study was to investigate the item discrimination /difficulty of each item in chemistry achievement test and to find out whether the guessing ability of each student has relationship in answering chemistry achievement test. The study adopted a descriptive survey design. A sample of 250 participants was randomly selected across twenty-five public secondary schools in Ibadan metropolis. Data were collected using“Secondary School Students chemistry Achievement Test (SSSCAT)” developed by the researcher. Data were analyzed using statistical package of the Social Sciences (SPSS24), polychoric correlation coefficient and JMetrik analysis. The results revealed that the item fitted 3-PL model the most because this model provided the least -2 log likelihood as a result of the model fit assessment conducted and it was discovered that 27 items out of the 30 items of the chemistry achievement test survived under IRT framework.