Dielectric metamaterials based on high-impedance silicon were designed for controlling the amplitude and phase of the terahertz waves. Here, the basic unit cells of the dielectric metamaterials are subwavelength silicon pillars, they have lower loss and thus have higher efficiency compared with the metal based metamaterials. As different silicon pillars with different geometric parameters and orientation angles will have different transmission amplitude and phase when the terahertz wave passes through them, nearly arbitrary spatial amplitude and phase distributions can be achieved by designing the geometric parameters and orientation angles of the silicon pillars at different positions, which allows to fully control the shape of the transmitted terahertz wavefront. In experiment, three different meta-gratings were designed using the silicon based structures in which the diffractive orders and their numbers can be arbitrarily controlled. Such dielectric metamaterials are very easy to design and fabricate, making them very promising in developing low-loss terahertz functional devices.