The present study adopted a theoretical model, suggesting that number sense consists of elementary number sense, conventional arithmetic and algebraic arithmetic , to trace the development of students' early number sense. Two hundred and four 1 st grade students were individually tested on five time-points. Data analysis suggested that number sense follows a linear growth rate and six groups of students were identified that follow different growth patterns. Two of the groups had a low initial value, two had a moderate initial value and two had high initial value. Systematically, one of the two groups with the same initial value exhibited a greater growth rate. The gap of the groups with same initial value in algebraic arithmetic progressively increased across the five time-points of the study.