Knowledge tests of health and physical fitness provide various levels of information about the factors they measure. Recently Fat Knowledge Test (Kang & Lee, 1994) was developed to investigate how many Korean young children have a correct knowledge on the obesity and to identify the test items which effectively discriminate between normal and obese children. The purpose of this study was to examine the appropriateness of the Fat Knowledge Test (FKT) by analyzing item and test informations and differential item functions between male and female and between normal and obese youths. The subjects for this investigation were secondary school male and female students at a large urban area in Seoul. FKT consisted of 15 true-false items was administered to 774 students. 343 out of 774 subjects were turned to be obese. Two-parameter logistic model using the PC-BILOG was utilized to estimate item and examinee ability parameters. ΔMH index by Mantel-Haenszel method and SA and UA index by Raju method were analyzed to test differential item function, using MHDIF and IRTDIF programs, respectively. The unidimensionality assumption was tested using a common factor analysis with a varimax rotation. This result provided strong evidence of the unidimensionality of the test on the basis of the criteria (Reckase, 1979) that the first factor should have more than 20% of the total common variance. Most items of FKT had good fit values, considering that the values of this residual greater than 2.0 were regarded as misfitting items (Mislevy & Bock, 1990). The items with a poor fit was items 1, 2, 4, 12, and 13. Results of item and test information analyses revealed that items 9, 11, and 14 provided maximum information at -0.80＜θ＜-0.25 level. The other items produced low item information at the all range of θ, especially at θ=-0.25 which indicated the cut-off point discriminating between normal and obese youths. However, FKT provided maximum information at the -0.80＜θ＜-0.20. It was revealed that FKT was appropriate to discriminate the students pertaining to θ=-0.20 level. On the basis of differential item function analysis, the 8 items (1, 5, 6, 7, 8, 9, 10, and 15) among 15 of FKT for the secondary school students showed statistically significant differential item function. However, the tentative contents analysis did not reveal why the differential item function exist in most of 8 items. It was concluded that mean differences of FKT scores between normal and obese students were not caused by the differential item function but the difference of students` abilities.