In this paper, we present a simple in-plane micro check valve using a hydrogel valve core fabricated by photolithography. Compared to early micro check valves with multilayered structure, this simple structured valve is much easier to fabricate and operate. Experimental results show that this check valve performs better than most existing in-plane micro check valves, with nearly zero forward cracking pressure and reverse leakage. The forward fluidic resistance of the valve is easy to control by adjusting the width of valve neck. We believe this type of micro check valve could have great use of flow control in integrated microfluidic systems and generate more potentials for MEMS to be utilized in real application.