The precision of the ultrasonic flowmeter with small tube diameter and frequencydifference method is affected by several factors,so to gain more accuracy is difficult. In this paper manyfactors affecting the precision of measurement with the ultrasonic flowmeter with small tube diameterand frequency difference method are analyzed mathematically. Being based on that and combining withthe practical experence of developing this flowmeter for several years, the measure and method forincreasing precision of measurement have been proposed.