One must do ultrasensitive leak detection during ultrahigh-vacuum III (UHV) evacuation, especially Just before the device is sealed off from the vacuum System, to guarantee the longevity of the sealed high-vacuum or even UHV devices with small volume. A quadrupole mass spectrometer (QMS) with an UHV evacuation system can be used under accumulation mode to do the testing. Possible accumulate modes, as well as their advantages and shortcomings, are Studied experimentally and discussed in this paper. We found that the opening action of the metal valve during accumulation mode always severely affects the height of the peak indicated by QMS and causes Considerable errors. If we determine the leak rate by the peal, area instead of the peak height, the situation is much improved. This method has proven quite useful in ensuring the tightness quality for complex sealed UHV devices with small volumes. Ultrasensitive leak detection has been carried out for such real evacuating devices, and a leak rate of 2 X 10(-14) Pa(.)m(3)/S was detected, which is far lower than its dynamic mode and the detection limit of the current advanced commercial leak detectors. (c) 2006 American Vacuum Society.