© 2020, Ozgen Korkmaz. All rights reserved.In this research, the effect of an item’s type and cognitive level on its difficulty index was investigated. The data source of the study consisted of the responses of the 12535 students in the Turkey sample (6079 and 6456 students from eighth and fourth grade respectively) of TIMSS 2015. The responses were a total of 215 items at the eighth-grade level (115 multiple-choice and 100 open-ended) and 178 items at the fourth grade level (93 multiple-choice and 85 open-ended) in the mathematics test. In the present study, the difficulty indices for the items of different types and cognitive level were calculated firstly. Then, the item type was defined as a dummy variable and multiple regression analysis was applied to test for the predictive effect of the type and cognitive level of the items on its difficulty index. In the study, it was determined that both the type and the cognitive level of the item had a statistically significant effect on its difficulty index. Compared to multiple choice items, it was detected that students had more difficulty in open-ended items. The effect of item type on difficulty index was found to be higher in eighth grade items than the fourth-grade ones. It was ascertained that difficulty index moved toward zero as its cognitive level increased at both grade levels. However, the effect of cognitive level on the difficulty index was higher in the fourth-grade items compared to the eighth grade items.