Abstract: The Differential Image Motion Monitor (DIMM) is a good instrument for seeing measurement and be commonly used.A three-sub-aperture DIMM built by Yunnan Observatory has been used for site testing at the Gaomeigu station since 1994. Some factors influencing the final results of seeing measurement are discussed.First,the Frield parameter r₀ is correlated with the size D of the telescope,the diameter d and the separation S of the Sub-apertures used in the seeing measurement.In principle,D≥3r₀ and K=S/d ≥2 are required.If the seeing=0.″6,i.e. r₀=18.7cm,then D≥56cm is needed.However,the diameter of the site testing telescope normally is less than 40cm,and some K<2.These instruments have the effect to the accuracy of r₀. Second,the sampling rate which was discussed by many authors,the observing year,season,date and time are also related to the measured result since the seeing is variable with long and short time scales.20 and 8ms exposure time were employed and the comparison is presented. Different criterions are used to calculate the image center of the star.There are no obvious differences for different criterions. It is needed to pay attention to the data processing.Some mistakes exist in Waldie's (1988) equations and the final results will be 30%～40% better form these equations than that by using corrected equations.