To realize a large scalable optical phased array and beam steering without grating lobes, we design a miniaturized dielectric optical nanoantenna operating at 1550 nm with high efficiency. The nanoantenna is trumpet-shaped with a footprint and composed of silicon in the subwavelength order. Light is fed from the bottom of the nanoantenna by a silicon waveguide and then radiated into free space efficiently. Because silicon is nearly transparent to the wavelength of 1500 nm, absorption loss caused by the material is very low and the nanoantenna has a high radiation efficiency. A three-dimensional electromagnetic simulation software is used for simulation and optimization. The final footprint of the designed nanoantenna vertical to the radiation direction is 0.82 μm×0.28 μm. The results of simulation and optimization indicate that the nanoantenna has good radiation characteristics at 1550 nm with a gain of 6.1 dB, an efficiency of 82.7%, a good symmetry of radiation pattern, emitting vertically to the nanoantenna plane and no bidirectional radiation.