This study aimed to investigate students’ mathematical competence in learning relationships between units according to the students’ performance in a SUKEN test of Level 6. A total of 139 students were selected as our target group and involved as examinees. Research instruments include students’ answer sheets, test item analyses, and textbook analysis. SUKEN test is a mathematical proficiency test used to identify related issues to improve teaching practices. The results from the first phase showed that there were 24 examinees or 17.27 percent of them had been successfully passed the passing criterion as 70 percent of the total marks 100. However, there was a lowest percentage (33.21%) of examinees showed that they were able to answer correctly in the questions related to the content domain of Quantities and Measurement compared to other content domains. On top of that, only 38.49 percent and 31.09 percent of the examinees possessed their competencies in content knowledge and the method of application respectively while they answered the Quantities and Measurement questions. Besides, the results of in-depth analyses from students’ answer sheets revealed that there were seven different groups of answers by analyzing examinees' responses in terms of their reasoning skills to support their responses. Examinees were found to have problems finding the relationship between cm3 and m3, whenever they have to use a relationship in three dimensions. Therefore, teachers are suggested to use the geometry model to assist students in understanding the relationships between the units.