Currently many National Metrology Institute (NMIs) as well as advanced calibration laboratories are using piston gas flow standards with mercury sealing method for gas flow calibration in the low pressure range. The flow in the small range is about cc/min. In this work, the low pressure gas flow calibration system at VMI is presented, designed and manufactured by the Taiwan National Metrology Institute, CMS/ITRI. The flow range is within (0.002 ÷ 24) L/min. The uncertainty of the reference system is assessed against the ISO/IEC Guide 98-3:2008 document, Uncertainty is evaluated from individual influence sources such as category A and B assessments. The standard uncertainty/relative standard uncertainty and degrees of freedom of the sources can be evaluated individually and then combined to produce a composite standard uncertainty/combined relative standard uncertainty and an effective degree of freedom. Finally, the relative expanded uncertainty is obtained by multiplying the relative standard uncertainty associated with a coverage factor at the 95% confidence level of the measurement result.