In spite of the voices to meet basic learning needs since the World Conference on Education for All in 1990, Thailand still faces the problems of low performance in mathematics at primary school level, especially in the area of ability in mathematical problem solving. This paper attempts to reveal the cause of students' low achievement through analysis of the levels of their abilities, which are classified into the five stages, i.e. reading, comprehension, transformation, process skills, and encoding. Comparison was made between students in Bangkok and Samutsakhon Province. There are five questions to interview forty Grade five students. The data suggested that most of students' errors occurred at comprehension level for structured questions while the errors for multiple choice questions occurred at the transformation level. Good performers' errors did not occur at reading level, but poor performers' errors occurred mostly at comprehension level.