One year was needed for the rice-field eel,Monopterus albus,to reach their sexual maturity when cultured in cage and they were all female at the age of 1 year.The gonad index of rice-field eel cultured in cage was roughly increasing in the first year until they reached sexual maturity and the maximum of the gonad index of M.albus was 14.33±2.07,when eel reached sexual maturity,the concentration of serum estradiol reached its maximum （1 233.52±126.07 pg/mL） and the concentration of serum testosterone reached its maximum,(13.99±1.54) pg/mL,in May,a month before eel reached their sexual maturity.