Abstract: In order to determine the law of annual cycle of the structure of thyroid in cobra,we chose 84 mature healthy male Naja naja Atra bought from Suichang,Zhejiang Province,divided them in 12 grougps,and pulled out their thyroid during the period of 9:00-11:00 a.m.on around the 20th days of every month to be studying materials.The thyroid pulled out were cut in sections according to common practice,The largest section received H.E.stain.The changing of struction of thyroid was observed and compared every month.The total number of thyroid follicle,pinosome in follicle cavity and columnar epitheliem in each specimen was counted respectively.Those numbers were taken as indexes,Their laws of annual cycle were counted by way of circular distribution statistics.The respective relationship between cobras activity grade and total number of follicle,columnar epithelium and pinosome was counted by way of χ2 tendency test.The correlation coefficient between the total number of follicle,columnar epithelium and pinosome was counted by way of linear-correlation.The results came out as follows:(1) The total numbers of follicle,pinosome and columnar epithelium all showed the annual cycle change was similar to the single-peak curve,The appearance of peak figure of follicle number was on June 17,The 95% reliability section of peak figure of whole cobra was from May 6 to July 28.The peak figure of pinosome number was on May 30,The 95% reliability section of peak figure of whole cobra was from May 17 to June 13.The peak figure of columnar epithelium numberon May 27.The 95% reliability section of peak figure of whole cobra was from May 15 to June 9.The peak figure of the total numbers of follicle,scolumnar epithelium and pinosome in cobra thyroid appeared later than that of the plasma TSH concentration,but earlier than that of the plasma T3.T4 concentration.This indicated that cobra plasma TSH is the promoter of the structural change in the thyroid of cobra and that the synthesis and secretion of T3,T4 results from the maturity of follicle epithelium.2) There were very significant positive correlation between the total numbers of follice,columnar epithelium and pinosome in cobra thyroid,and the total numbers of follicle,columnar epithelium and pinosome,which link obviously with the activity grade of cobra.(3) During hibernation (the third ten days of November to the third ten days of February the next year),the follicle epithelium of thyroid in cobra presented the form of 2-6 layers,with a single layer at the active stage (in the other months).