Polymer-inorganic semiconducting composite (MCTP-Fe3O4) particles were fabricated by loading nanosized Fe3O4 on the microporous covalent triazine-based polymer (MCTP) through a chemical coprecipitation method and then applied to an electrorheological (ER) material. The structural and morphological images of MCTP-Fe3O4 composite were examined by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Their magnetic property was also investigated by vibrating sample magnetometry. The chain structure formation of MCTP-Fe3O4 dispersed in silicone oil under an external electric field was confirmed using an optical microscope. The ER fluid based on MCTP-Fe3O4 was processed by dispersing the composite particles in an oil medium, and for comparison, an ER fluid based on pure MCTP was also prepared by the same process. The ER performance of two different ER fluids was scrutinized by a rotational rheometer, which demonstrated that MCTP-Fe3O4 showed better ER characteristics than MCTP-based ER suspension.