Asthma is one of the most common chronic diseases in the world. It is estimated that around 300 million people in the world currently have asthma. The prevalence of asthma is increasing in most of countries, especially among children. The burden of severe asthma is considerable high in the Middle East courtiers including Iran. This study to investigate the effect of patients' knowledge about Inhaled Corticosteroids (ICS), attitude and health beliefs toward ICS, and behavioral intention to adhere to prescribed ICS in adult asthmatic patients in Yazd city, Iran. A cross sectional study was conducted from August 2008 to January 2009 in three private allergy and asthma clinics, located in Yazd city of Iran, using Structured face to face interviews using a questionnaire by the researcher. The results showed that the majority of patients (55.5%) were not adherent to their prescribed ICS. Patients’ knowledge toward ICS did not have effect on medication adherence behavior, while patients with positive attitude toward ICS were better adherent with their medication. Linear regression model identifies intention to comply with treatment and positive attitude toward ICS as predictors for adherence behavior. This study shows the relationship between medications beliefs, attitude, behavioral intention, and medication adherence. A better understanding of patient's medication beliefs, and attitude and their effect on compliance may help health care system to promote adherence.