Modular concepts open a new way to create customized integrated microfluidic devices for the changing needs of users. However, easy-to-follow modular construction at the micro-scale remains a crucial challenge. Here, we present a one-step liquid molding based modular method. Liquid molding was coupled with standard SU-8 lithography to fabricate the connection adapters and the intricate micro-flow networks of modules. The connection adapter in each of the modules with three-dimensional topographic structures bridges the gap between the macroscopic world and the microfluidic network. Analogous to electronic circuits, individual functional components were assembled together via the standard fused silica capillary tubing in series or parallel, forming a leak-free integrated whole. The modular microfluidic circuits were further applied to pathogenic bacteria detection and parallel droplet generation. Via these applications, we demonstrated that modular circuits can be easily assembled and disassembled, thus enabling easy reconfiguration. Additionally, the ability to incorporate components made from different materials was exhibited.