We present two PDMS-based gas purge valves for general lab-on-a-chip applications. The valves are devised based on a three-layer configuration comprising a top layer for fluid channels, a membrane, and a bottom layer for gas channels. The pneumatic valves work as a normal gateway for fluids when the membrane is bulged down (open state) or up (closed state) by vacuum or pressure, respectively. In a closed state, the residual gas in front of a fluid can be eliminated through a small notch or a permeable PDMS membrane by simply pumping the fluid. The purge valve with a small notch, termed surface-tension enable valve (ST valve), can resist pressure under 5.5 kPa. The liquid is retained by the surface tension force resulting from the surrounding hydrophobic walls. In contrast, the purge valve with vacuum grooves above the [fluid channel, termed gas-permeation enable valve (GP valve), can resist pressure higher than 5.5 kPa. Based on permeation principle, the unwanted gas can slowly escape from the fluid channel through the PDMS membrane. The unique purge valves enable users to passively align fluids at the desire locations without actively sensing or having a feedback loop. A mixing process was successfully performed with the GP valves, showing their potential applications in microfluidics.