Abstract: A novel highly accurate, ultrasensitive leak-detector with a lower detection limit of 5×10^-16Pa·m³/s, to be used in fabrication of vacuum electronic devices, was developed based on accumulation comparison method. The original work included the accumulation chamber of the vacuum system, a lab-built flow-meter with a lower flow-rate limit of 5 × 10^-16Pa·m³/s used as reference standard and a non-evaporable getter (NEG) pump.In the accumulation chamber, which could be independently pumped with NEG pump, the background partial pressure of trace helium was minimized with helium adsorption-resistant coating on the wall. Depending on the leakagerate of leaked helium, the partial pressure of the accumulated helium was high enough for the measurement with a quadrupole mass spectroscopy (QMS). The test results show that the leak detection uncertainty was in 5. 3% ~13%, in the range from 10^-12 to 5×10^-16Pa·m³/s.