This paper describes a simple and effective method to fabricate oblique or curved 3D microstructures with high uniformity and manifold shapes, utilizing backside 3D diffuser lithography. A negative photoresist spin-coated on the metal-patterned glass substrate is exposed from the backside where an optical diffuser is located and used to randomize the incident ultraviolet (UV) light, which creates the self-aligned curved 3D microstructures. The various 3D microstructures can be obtained simply by adjusting the key process parameters of this method such as the type of diffuser, the UV exposure dose, the opening width of the patterned metal on the glass substrate, etc. Through these parameter studies, a microball lens with a sag of 10 µm to 115 µm, a cylindrical microlens, a 3D planar microlens with circular and biconvex structures and an inverse-trapezoidal microstructure having different sidewall angles of 45°, 51°, 56° and 72° were successfully fabricated. The proposed method can be extensively applied for fabrication of micro-optical components due to its simplicity and versatility.