Postgraduate students need to be equipped with self-determined learning skills in order to meet the demands of higher degree and employment in the fourth industrial revolution (IR 4.0) environment. However, the lack of analysis conducted on item level for validating the measurement of postgraduate students' self-determined learning skills prompted the development of The Postgraduate Self-determined Learning Questionnaire (PSLQ) to fill in the research gaps. This paper aimed to assess the psychometric properties of the newly developed 42-item instrument through a mixed method approach as no study has evaluated the psychometric properties of PSLQ at the item level through Rasch model analysis. Quantitatively, 440 postgraduate students were sampled from a higher education institution in Malaysia to answer the questionnaire. Qualitatively, 10 respondents from science and non-science disciplines were interviewed. The paper reports findings on the psychometric properties of PSLQ through Rasch Rating Analysis and participants' perspectives on the item validity. The results revealed that the items in PSLQ demonstrated good psychometric properties for the measurement of self-determined learning skills of postgraduate students. Implications of the study were discussed in this paper. Future studies can further investigate the psychometric properties of PSLQ according to students' discipline of study.