The author detected O-type foot and mouth disease maternal antibody came from 0,1,7,14,21,28,35 and 42-day-age piglets born by 6 pigs.The immunity effect of first and second immunity also was observed.The results showed that maternal antibody didn’t existed in 0-day-age piglet without eating colostrums,and the highest level of it occurred in second day,then it gradually degraded.1～14-day-age piglets were protected by swine pest maternal antibody,some piglets also survived after 21-day-age.The piglets were first immunized in 28～35-day-age.The protected rate was lower than 70% in the 21st days and 86.7% after 8 days.The second immunity occurred in the 31st days after the first immunity.The immune antibody could sustain high level within 4 months after the second immunity,then the immunity protected rate gradually decreased,it couldn’t effectively protect the piglets after 5 months,so breeding pigs should be immunized again.