Polymer microfluidics has received considerable attention due to their low cost, remarkable biocompatibility, and high flexibility when compared to glass and silicon devices. However, the fabrication process of all-polymer devices can be complicated. In particular, different types of polymers possess different properties in terms of surface chemistry and hydrophilicity, making device assembly a challenging task. In this paper, we demonstrate the fabrication of an all-polymer device through the investigation of the essential surface treatment methods. A hybrid SU-8-SU-8-polydimethylsiloxane (PDMS) sandwiched structure is used in this research. Both untreated SU-8 and PDMS are hydrophobic and they have different surface chemistry properties, so surface modifications are necessary. Three critical surface treatment steps are used in our process. The first step is to treat the first SU-8 layer with low-power (10 W) oxygen plasma, making its surface hydrophilic. This step enables the uniform coating of the second SU-8 layer. The next surface treatment is on the second SU-8 layer. Both oxygen plasma (40 W) etching and diluted 3- aminopropyltriethoxysilane (APTES, a silane solution) coating are needed. APTES introduces amine (Si-NH2) groups on the surface. The last treatment step is to introduce silanol (Si-OH) groups on PDMS using oxygen plasma. These surface treatment steps are critical in the fabrication process and can determine the quality of the final device.