An analysis is made of the differences between automatic and manual daily mean meteorological measurements (24 times per day and 4 times per day, respectively), and the ttest method is used to determine whether there is distinct difference between the means of two samples. The analysis of differences between 24timesperday and 4timesperday observations show that there is remarkable deference in relative humidity for about half stations, and there is remarkable deference in daily temperature, pressure and 2min wind speed at a confidence level of 005 for 15%, 0 and 69% of stations, respectively. Moreover, there are evident local characteristics in temperature and pressure differences. The analysis of differences between 24timesperday automatic observation and 24timesperday manual observation indicates that there is remarkable deference in daily mean temperature and pressure for 15%, 0 and 6.9% of stations, respectively, and in relative humidity and 2min wind speed for 762% and 787% of stations, respectively. It is obvious that the impact of two kinds of observation instruments is universal and prominent