HbA1c is the major fraction of glycated hemoglobin, and used primarily to identify the average plasma glucose concentration over prolonged periods of time, mostly 2-3 months. It is used as markers for the diagnosis and monitoring of diabetic patients and increasingly used a marker in health screening check up for general populations. In this study, HbA1c was measured with Cobas integra 800 (Roche Diagnostics, Mannheim, Germany), using immunoturbidimetry principles. We established the reference interval for HbA1c with Cobas integra 800 and evaluated its significance. The study subjects were 36,140 (male 57.5%, and female 42.2%) who visited the Health Promotion Center of a tertiary care center in Seoul for health checkup from January to September, 2008. HbA1c levels were measured with immunoturbidimetric method. Statistical evaluation was done with SPSS. Comparison between male and female was checked with Mann-Whitney test, and among age groups with Kruskal-Wallis test. Reference interval for HbA1c was from 4.8% to 6.1%. There was no significant difference between male and female with Mann-Whitney test (P=0.539). There was significant difference among age groups with Kruskal-Wallis test (P<0.05). Reference interval for HbA1c with Cobas integra 800 was 4.8~6.1%, which was different from conventional one, 4.4-6.4%. Establishment of reference interval for each principle is needed.