The aim of this study is to explore the mathematical problem posing performance of students in free structured situations. Two classes of fifth grade students (N = 30) were randomly assigned to experimental and control groups. The categories of the problems posed in free structured situations by the 2 groups of students were studied through inductive and deductive content analysis and coded in categories before and after a 5-week treatment. The treatment for the experimental group was the Interlocked Problem Posing Instruction in which student cognitive engagement was emphasized, and for the control group, it was Traditional Problem Posing Instruction. The results revealed that the experimental group students posed more solvable, reasonable and start-unknown problems than the control group students after the treatment.