The purposes of this study were to examine the Hong Kong 5th and 6th graders' performance on the number sense three-tier test, the differences between 5th and 6th graders, and possible misconceptions. The number sense three-tier test included a content-tier which assesses content knowledge of number sense; a reason-tier, which assesses a reason selected for the first-tier response; and the confidence-tier, which assesses how confident the students are in their answers to the first two-tiers. A total of 125 fifth graders and 942 sixth graders from Hong Kong joined the test. A quantitative method was used to analyze the data and the data was collected through the on-line test. Data showed that the correct percentage (confidence index) on the first two-tiers (content-tier and reason-tier) for fifth and sixth graders are 56.6% (4.49) and 54.0% (4.27), respectively. The fifth and sixth graders all performed the best on “Being able to recognize the number size” and the worst on “Being able to judge the reasonableness of a computational result”. Results showed that part of students performed poor on number sense but with extreme high confidence indicating that they have significant misconceptions and some students may lack number sense. The finding of this study also confirmed that a number sense three-tier test can be used to mitigate the weakness of a two-tier test. Educational implications of the findings will be discussed.