We designed, fabricated, and characterized a thermo-optically tunable compact ( $10\mathrm{\mu m}\times 10\mathrm{\mu m}$ ) silicon photonic crystal (PhC) light modulator that operates at around $1.55\mathrm{\mu m}$ for TE polarization. The operational principle of the device is the modulation of the cutoff frequency in a silicon-based line defect PhC. The cutoff frequency is shifted because of the thermo-optic tuning of the silicon refractive index, which is realized by localized heating on the PhC. The modulator is formed by a triangular lattice array of cylindrical air holes on a silicon-on-insulator wafer. Optical characterization was carried out, and the result clearly showed thermo-optic tuning of the cutoff frequency at around $1.55\mathrm{\mu m}$ .