This paper was designed to investigate the causes of child hearing loss which is essential to language development especially in young childhood. Accurate history taking, precise otolaryngological examination and impedance audiometry were performed on 185 hard of hearing children under age 5 during last 8 months and investigated its causes. The results were as follows ; 1) Infectious diseases were the most common causes of sensorineural hearing loss(44/123 cases, 30.1%), and mother's condition during pregnancy the second, and birth injury or head trauma the third. 2) Acute or chronic ear diseases were the most common causes of conductive hearing loss(32/62, 51/6%), frequent URI the second(19/62, 30.6%), and adenoid vegetation the third(7/62, 11.3%). 3) The majority of causes of child hearing loss under age 5 was exogenous(179/185 cases, 96.8%), and the minority was endogenous(6/185 cases, 3.2%).