Three dimensional and four directional carbon fiber reinforced SiC matrix (C_f/SiC) composites were fabricated by precursor infiltration and pyrolysis process with ameliorated interface and low porosity employing a new precursor, liquid polyvinylcarbosilane (LPVCS). LPVCS is a novel precursor with active Si-H and –CH=CH₂ groups with relatively high oxygen content (~7.3 wt.%). The mechanical properties improvement led by ameliorated interface and low porosity was reasearched. Samples with ameliorated interface and low porosity employing LPVCS showed better mechanical properties than those employing ploycarbpsilane (PCS). The flexural strength and fracture toughness of the C_f/SiC composites fabricated with PCS were 301 MPa and 11.2 MPa.m_1/2, respectively, whereas the equivalent values of the samples fabricated with ameliorated interface and low porosity were 442 MPa and 26.1 MPa.m_1/2, respectively. Employing LPVCS for the first infiltration and pyrolysis cycle formed suitable interface, employing PCS as precursor for the left cycles formed relatively low oxygen content matrix, then employing LPVCS for the last several cycles formed low porosity, this the main reason of mechanical properties improvement.