This paper investigates the relationship between national culture and organisational culture in the context of medium and large size organisations in Iran. After discussing the theoretical framework of national culture and organisational culture, the paper highlights some managerial perspectives in Iran. Using survey methodology the national culture and organisational culture dimensions of several organisations in Iran were investigated. Questionnaires were personally administered as well as by email. 225 questionnaires were returned out of 626, of which only 221 were used for further analysis. Questions related to national culture were based on Hofstede’s national culture dimensions using the Dorfman and Howell (1988) scale and questions in the organisational culture section were directly borrowed from the Organisational Culture Assessment Instrument (OCAI). The analysis indicated that, in the case of medium size organisations, there is a significant relationship between national culture and hierarchy culture whereas, in the case of large size organisations national culture has a significant relationship with market culture. It is recommended that researchers should consider the firm size as a moderator for better understanding the linkage between organisational culture and national culture in the case of a developing country.