This study aims to develop and validate a unidimensional scale of collegiality to assess EFL (English as a foreign language) teachers' collegiality competence. Rasch rating scale model (RSM; Andrich, 1978) was used to analyze the scale with regard to model fit, use of response categories, and reliability. To this end, a total of 202 Iranian EFL teachers were administered the newly-developed scale. Results showed that the five-point category scale had an optimal function and enjoyed acceptable reliability. Adequate model-data fit was achieved after removing several misfitting items. The fit of the data to the model was empirical evidence for the unidimensionality of the scale.