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We propose a method to quantify the true leak rate of micro to nano-liter packages using the helium mass spectrometer. A new concept called ldquopreprocessing timerdquo is introduced to take into account 1) the instability of the helium mass spectrometer during the initial part of its operation and 2) the contribution of viscous conduction to the total conduction. The proposed method utilizes the complete profile of the apparent leak rate measured by the mass spectrometer and determines the true leak rate by performing a nonlinear regression analysis. The method is implemented successfully to measure the true leak rate of micro-electro-mechanical system packages. The validity of the proposed scheme is corroborated experimentally.