A ratio type estimator of population mean has been proposed by using auxiliary information efficiently under ranked set sampling (RSS). The expressions for bias and mean square error (MSE) of the estimator have been obtained. The proposed estimator has been compared with existing estimators of population mean. It has been shown that the proposed estimator is more efficient than the estimators suggested by Samawi and Muttlak (1996), Bouza (2008) and Jemain et al. (2008).  The results of the proposed estimator have been verified through simulation study.