Curriculum development has always been dealt as one of the most important areas of language learning and teaching by educational institutions. Null curriculum has been viewed as one of the significant kinds of curriculum types which is important due to its absence, being left out or overlooked but should not be disregarded. For this reason, a 48-item questionnaire for null curriculum in ELT with a special focus on twenty-first century skills was developed and validated by the application of the Rasch model. The Rasch model was employed to decide whether the scores of an instrument are purposive, meaningful and significant. Rasch analysis using Winsteps software version 3.73 was used to determine the dimensionality, use of response category, sample appropriateness and reliability of the scale. The test was administered to 80 university instructors teaching at BA level in different universities in Iran. The results revealed that the Rasch model fits the test after deleting six items from the scale and a suitable reliability was confirmed. This indicates that the questionnaire is potentially valid and can be used as a tool for measuring aspects of null curriculum focusing on twenty-first century skills.