In an interview setting, 21 6th-graders from four public schools in south Taiwan were asked a series of questions, related to whole and decimal numbers, designed to assess their number sense. Eight students were randomly selected from both low and middle performance levels, and five were selected from a high-performance level, according to their mathematical capabilities. Results indicated that, regardless of performance level, very few number sense strategies (e.g. using benchmarks, estimation or numbers of magnitude) were used. The evidence also revealed that Taiwanese students tended to apply rule-based methods and standard written algorithms to explain their reasoning. Their inclination to use paper-and-pencil procedures narrowed their thinking and reasoning powers: this heavy reliance on written algorithms seemed to be a major impediment to the development of number sense.