Based on the epsilon-near-zero (ENZ) effect of indium tin oxide (ITO), we numerically demonstrate a high efficiency ITO phase/intensity modulator by exploiting ultra-thin silicon strip waveguide configuration. Heavily n-doped indium tin oxide is used as the semiconductor together with p-doped silicon and hafnium oxide (HfO2) to form a MOS waveguide. Due to the special feature of the ultra-thin silicon waveguide structure, the propagating transverse electric (TE) mode is less confined to the silicon core and penetrates deeper into the cladding layer, which will enhance the interaction between the active material and the optical mode. The combination of the ultra-thin silicon strip waveguide and ITO material exhibits high modulation efficiency together with broad optical bandwidth. When the modulator operates as a phase modulator, the effective refractive index change can reach the value 8:95x10-3 for the light wavelength λ = 1550 nm when the applied voltage is 6 V. Thus, the phase shifter length which can induce a π phase shift is supposed to be only about 97 µm, giving a corresponding VπL of 0.58 V∙mm. The effective index change even keeps > 7:32 x 10-3 with the wavelength increasing from 1300 nm to 1800 nm, indicating the broad modulation bandwidth. Meanwhile, the modulator can also operate as a variable optical attenuator or an intensity modulator. The modulation depth (MD) is about 0.074 dB/µm at 9 V when the wavelength is 1550 nm. This device confirms electrical phase shifting in ITO enabling its use in applications such as compact phase shifters, sensing, and phased array applications for LiDAR.