Infant  mortality  has  an  important  place  in  mortality  research ，which  is  the  beginning  of  the mortality  force changes by age and which also affects later － life mortality change． Analysis of the past censu- ses demonstrates that a large gap exists between the census － reported infant mortality rate and the actual rate as a result of underreporting． By analyzing the relation between infant mortality and economic development of 174  countries of the world，comparing census data with health statistics ，and examining provincial variations in infant mortality ，it is found  that  there  is  significant  underreporting  in  infant  mortality  in  the 6th  census． Since  errors in the reported mortality of the population aged 1 － 4 are relatively small ，we first adjust it by areas ，then adjust infant mortality according to the steady relation between infant mortality and mortality of the population aged 1 － 4． The  adjusted  national  infant  mortality  rate  stood  at 17． 27‰  in 2000 ，implying  that underreporting rate of the infant mortality in the 6th census is about 78% ．