The impacts of SWCNT and MWCNTOH on the antioxidant defense system of juvenile goldfish (Carassius auratus) were studied by semistatic exposure method. SWCNT and MWCNTOH were found to induce histopathological and oxidative damages in liver. The results showed that the antioxidant enzymes SOD and CAT activity were inhibited significantly in the liver of goldfish exposed to 10, 100 and 1 000 μg/L individually for 28 d. Individual exposure to SWCNT or MWCNTOH at 14 d resulted in histopathological gill changes as follows: aneurisms, oedema in the secondary lamellae and hyperplasia in the primary lamellae. SWCNT and MWCNTOH exposure was also found to induce necrosis and vacuolization in liver at the high dose of 1000 μg/L. However, no changes were observed in liver at the low SWCNT or MWCNTOH exposure level (dose of 10 μg/L) compared to control. No lethal effects were observed during the test. With the increase of exposure days, two carbon nanotubes could inhibit SOD and CAT enzyme, and the inhibition was obvious in the group with high concentration, and the antioxidant system of the liver was damaged. The results indicated that the long term impacts of carbon nanotubes to fish need to be further investigated.