This study examined the existence of latent classes in TIMSS 2015 data from three countries, Singapure, Turkey and South Africa, were analyzed using Mixture Item Response Theory (MixIRT) models (Rasch, 1PL, 2PL and 3PL) on 18 multiple-choice items in the science subtest. Based on the findings, it was concluded that the data obtained from TIMSS 2015 8th grade science subtest have a heterogeneous structure consisting of two latent classes. When the item difficulty parameters in two classes were examined for Singapore, it was determined that the items were considerably easy for the students in Class 1 and the items were easy for the students in Class 2. When the item difficulty parameters in two classes were examined for Turkey, it was found that the items were easy for the students in Class 1 and the items were difficult for the students in Class 2. When the item difficulty parameters in two classes were examined for South Africa, it was ascertained that the items were a bit easy for the students in Class 1 and the items were considerably difficult for the students in Class 2. The findings were discussed in the context of the assumption of parameter invariance and test validity.