Recently, chip based fabrication of curved microstructures, such as microsphere, microfiber and microtube, has attracted much attention owing to their many advantages. In this paper, we have fabricated pH-responsive flexible microfiber using the microfluidic chip and ’on the fly’ photopolymerization method, and we characterized their mechanical and swelling properties. First, we have measured the swelling properties according to the pH variation. The PDMS based test apparatus was prepared, and we inserted the microfiber and measured the volume transitions. The shrinking and the swelling motions were measured according to the different sized (50, 80, 100 μm) fibers to investigate the size effect. The effects of acrylic acid (AA, pH sensitive monomer) concentration were examined also. As the fibers’ diameter increases, the response of volume transition was slow, and higher AA showed larger volume expansion to the application of basic solution. Second, we have measured the elasticity of microfiber according to AA concentration (weight %: 6, 11, and 22). The elasticity at the fiber having less AA concentration was larger, and this indicates that the fiber having lower AA concentration is softer. The advantages of our fabrication method are as follows; 1) the process is simple and cost effective, 2) we can functionalize the fiber and this can be applied to diverse microdevices such as sensors or actuators, 3) the size can be easily altered via the regulation of flow rate. Here, we showed the fibers’ mechanical properties, and we expect that the microfiber can be applied to the fabrication of artificial muscle.