This study examined changes in physicochemical indicators [K-value, lipid peroxide, and thiobarbituric acid reactive substances (TBARS)] in mackerel muscle during refrigerated storage at 4℃. Analyses were conducted every 3 days for 15 days. First, we visually observed changes in the mackerel. After 3 days, there were rapid changes in color and a rotten smell developed. The K-value of mackerel muscle was 4. 21, 19.00, 42.13, 51.71, 71.19, and 80.34% on days 0, 3, 6, 9, 12, and 15, respectively; lipid peroxide level also increased, to 5.34, 5.44, 6.59, 7.29, 8.32, and 8.71 nmol/ mL, respectively. TBARS increased from day 0 to 6, but did not change significantly after day 6 (0.98, 1.74, 2.73, 2.09, 1.86, and 1.93 mg Eq. malondialdehyde (MDA)/kg, respectively). The changes in K-value and lipid peroxide level demonstrated a loss of value as fresh food after day 6. The freshness of the stored mackerel had decreased by day 6 and mackerel stored for 9 days was less marketable.