The purpose of this study is to develop tests for monitoring the number sense skills of primary school students based on proficiency definitions and to compare their number sense skills according to gender, school type, and mother's educational level. The first stage addressed to test development in which anchor items were used for vertical equating of number sense tests across grade levels. In the second stage, the number sense skills were compared in terms of student characteristics. The participants were 2034 primary school students. Reliable and valid number sense tests each consisting of 20 items were developed that allow comparison among Grades 2, 3, and 4. The findings indicated that number sense skills were not developed sufficiently in early childhood and had different trends in terms of gender through grades. Moreover, private schools outperformed public schools and the gap became higher in Grade 4. The results also showed that mother's education is an important factor for children to have higher number sense skills. Number sense is a prominent predictor of mathematics performance and monitoring number sense progress helps to evaluate the effectiveness of mathematics instruction.