The High Order Thinking Skills Test Instrument (HOTs) was developed to measure the High Order Thinking Skills of Form Four students for the topic of Quadratic Functions in Additional Mathematics Subject in secondary schools. This study aimed to determine the validity and reliability of HOTs instrument through the Rasch Measurement Model. HOTs consisted of eight open-ended items with four constructs namely applying, analyzing, evaluating and creating. The sample consisted of a total of 73 (16-year-old) students from secondary schools in Lahad Datu district of Sabah. The results of the validity analysis found that the polarity of the items through the PTMEA-Corr value showed that all 8 items were > 0.00 (+). Through item suitability analysis, all items retained as eligible in one of the MNSQ outfit ranges, ZSTD Outfit and PTMEA- Corr. In the Principal component analysis (PCA), the value of Raw Variance Explained by Measures was 79.3% (excellent) and the value of Unexplained variance in the first contrast was below 15% indicate that the instrument has strong evidence of unidimensionality and high construct validity. Overall, the HOTs was found to have a very high reliability with a Cronbach’s alpha value (KR-20) of 0.90. Results also showed that HOTs has an excellent item reliability (0.98), very good person reliability (0.91), a high item separation value (5.63) and high respondent separation value (2.86). In conclusion, HOTs has good validity and high reliability in measuring High Order Thinking Skills in Quadratic Functions among Form Four students in secondary schools.