The aim of this study was to develop an instrument for assessing sixth-grade students' number sense skills in fractions and decimals. This study was conducted on 452 sixth graders (10-11 years old) from the western region of Turkey. The construct validity of the number sense test (NST) was examined via exploratory factor analysis (EFA) and confirmatory factor analysis (CFA). First, the draft NST, which consists of 36 two-tier items, was administered to 208 students for the EFA. Repeated EFA results indicated that the NST consisted of five number sense components with 25 items (understanding the meaning and magnitude of numbers, recognizing the effects of operations on numbers, using multiple representations of numbers, using reference points in numbers, and judging the reasonableness of a computational result). The NST, with factor loadings ranging from 0.60 to 0.87, accounted for 72.39% of the total variance. The composite reliability and Cronbach's alpha coefficients ranged from 0.88 to 0.92. The CFA was administered to an additional 244 students in the school. The results of the first- and second-order CFA revealed that the model fit of the NST was good (Chi-square/df = 2.04; RMSEA = 0.066; NNFI = 0.97; IFI = 0.98; CFI= 0.97; and S-RMR = 0.065). The results of the present study indicate that the NST is a valid and reliable instrument for assessing sixth-grade students' number sense skills in fractions and decimals.