Lithium iron phosphate (LiFePO4) based material is one of the most prospective candidates as a cathode material in lithium-ion batteries because of its lower cost, safer, and environmental benignity compared to lithium cobalt oxide (LiCoO2), which is commonly used for lithium-ion batteries manufacturing. However, its low conductivity is the obstacle of this material to solve, so that modification with the addition of silicon (Si) is expected to improve the electrochemical performance. Meanwhile, solid state reaction is considered simple and effective in LiFePO4 crystal growth process. Therefore, Si-doped LiFePO4 using solid state reaction in this research aims to study its structure and morphology as well as the effect of adding Si to its conductivity. The synthesis began with mixing LiH2PO4, Fe2O3, carbon black, and six-mole ratio variation of Si to LiFePO4 using agate with ethanol: acetone addition then dried in an oven at 80°C and heated at 550°C in a furnace for 6 hours under argon atmosphere and sintering temperature of 870°C for 16 hours with the same condition. The sample of 3% mole ratio performed the highest conductivity of all variations with 3.01 x 10-6 S.cm-1, and was identified as Li0.93Fe1.07P0.93O4Si0.7 with orthorhombic structure, Pnma space group (Ref. Code: ICSD 98-016-1792) with the highest peak at 2θ = 35.556° from XRD analysis with rectangular-like shape particle.