As the Islamic endowment (Waqf) has been practiced throughout the world, it is a challenge to measure the level of the generosity index on its practice. In fact, the awareness in performing Waqf among Muslims especially in the Generations Y and Z are important since it is valuable to the Muslims and concurrently can help those needy people. The study objective is to develop the generosity index among the two generations' classifications. The variables that have been selected in this study namely trust, altruism, personal characteristics, self-image, religious value, psychological benefits, social norms, personal satisfaction and commitment. Total of 450 questionnaires managed to be collected from Generations Y and Z respondents in Klang Valley area. Rasch Measurement Model was employed to analyze the data. The construct was found reliable with Cronbach Coefficient Alpha of 0.94. The statistical indicators such Point Measure Correlation (PtMea Corr) and Person Item Distribution Map (PIDM) revealed that all the two generations have different results for their generosity index. This can be relating where each generation having their own behavior traits in attitudes and intentions. The implication of this paper provides evidence of the usefulness of Rasch Measurement Model in identifying the Generations Y and Z generosity index on Waqf or named as Waqf Generosity Index (WGI).