Large-area and high-quality monolayer graphene was synthesized in order to fabricate a graphene-base highly sensitive strain sensor. A rapid LPCVD (Low Pressure Chemical vaper deposition) synthesis process of monolayer graphene was developed by using acetylene as a resource gas. To synthesize high-quality single-crystal graphene, the surface of copper substrate was strongly orientated to (111) crystallographic plane. By optimizing the concentration of acetylene gas by diluting hydrogen, the high quality of monolayer single-crystalline graphene film was successfully grown on the copper substrate. A strain sensor was fabricated using the graphene-coated Cu foils by applying the MEMS process and reactive ion etching (RIE). Then, the sensor was transferred onto a polydimethysiloxane (PDMS) substrate. Tree-dimensional bending test was performed to investigate the piezoresistive property of the patterned graphene nano-ribbon. It was confirmed that the highly sensitive strain sensor was obtained when the width of the nanoribbon was thinner than 70 nm.