This study was conducted to measure residual aluminum concentrations, affected by the flocculation process in water treatment, the raw and tap water of 14 major cities in Korea during January and February 1989. The results are summarized as follows ; 1. The Maximum aluminum concentration detected in the raw water was 0.171㎎/ℓ in samples collected in the Inchon region. 2. The Average aluminum concentration detected in the tap water of 14 cities 0.149㎎/ℓ. 3. The samples were categories by aluminum concentration in the tap water and the results indicated that aluminum concentrations between 0.1∼0.2㎎/ℓ showed the highest frequency of 53.5%. 4. Of the overall samples, 22.4% exceeded the WHO`s recommended guideline value of 0.2㎎/ℓ. The samples of the Chunchon region showed the lowest value whereas the frequency of concentration exceeding the WHO`s value was 50.0% in the Pusan region. 5. Among the three kinds of coagulation in the water treatment system treated with alum, without coagulants and with mixed coagulants, the average aluminum concentration of the distribution system treated with alum showed the highest value. 6. The aluminum concentration in January and February was 2.58㎎/ℓ fold higher compared to that of May in Seoul region.