A tunable optical add-drop multiplexer in silicon-on-insulator (SOI) is proposed by using thermo-optic effect. The device integrates a two-mode interference coupler with a set of tilted gratings, and its operation principle is based on multiple-mode interference principle and coupled-mode theory. The 3-dB bandwidth of add/drop wavelength is 0.5 nm in the reflection spectrum. Its central add/drop wavelength range is from 1550 to 1558.5 nm when the applied power is less than 14 W in the two-mode interference section, i.e. gratings section. The device is attractive in terms of tunable range of add/drop wavelength and design tolerance.