【Abstract】 Objective To consecutively measure the access flow (Qa) change in arteriovenous fistula during a hemodialysis session using ultrasound dilution technique in order to understand the appropriate period for Qa measurement in a hemodialysis session. Methods We consecutively monitored the parameters of Qa in arteriovenous fistula, cardiac output and cardiac index during a hemodialysis session at 30, 90, 150 and 210 minutes by using a Transonic HD02 hemodialysis monitor in 49 maintenance hemodialysis patients. The changes of Qa and the effect of hemodynamic changes on Qa were then evaluated. Results There were no significant differences among the average Qa values and the Qa values adjusted with mean arterial pressure (MAP), which were measured at the 4 time points. Qa correlated positively with MAP, so did the change of Qa with the change of MAP in a hemodialysis session. MAP was the independent impact factor of its respective Qa at every time point. After a hemodialysis session for 210min, Qa change was significantly bigger in the patients with ultrafiltration volume more than 5% of their dry body weight than those with ultrafiltration volume less than 5% of their dry body weight. Conclusion Qa can be measured at any time in a hemodialysis session, but not suitable for patients with significantly lower MAP during hemodialysis. For patients with the ultrafiltration volume more than 5% of their dry body weight, Qa measurement was not recommended after 150min or longer in a hemodialysis session.