Inhalation of radon gas and its decay products ranked as a main source of natural radiation exposure. It is considered as a major cause of lung cancer after cigarette smoking. There are some very important factors that strongly affect the indoor radon levels such as: indoor airconditions, ventilation systems, and meteorological factors. In this article, we determined the indoor radon concentration and studied the seasonal variations of indoor radon levels to evaluate the annual effective dose received by the inhabitants in Al-Kharj city, Saudi Arabia. For this study, 84 houses were chosen and RAD 7 detector was used to examine cumulative radon exposure concentration from September 2014 to November 2017. Our measurements showed an obvious difference between radon concentration in winter (24.33 ± 11.10 Bq/m 3 ) and in summer (14.54 ± 5.50 Bq/m 3 ), while the average concentration in spring and autumn was very much close to the average annual radon concentration (19.23 ± 8.13 Bq/m 3 ). The average annual effective dose was found to be 0.49 mSv/y. The study revealed that the average indoor radon concentration as well as the annual effective dose rate is found to be less than the action level recommended by ICRP.