Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force. Since its inception in 1970, optical manipulation has evolved into a versatile tool across many fields of science. Notably, the now widely employed optical tweezers invented in 1986 is a good example, which is in essence a strongly focused fundamental Gaussian beam. Although the optical tweezers remained as an important tool in optical manipulation, the shaped structured light such as an optical vortex beam also provides unusual light patterns and promotes exciting discoveries. This thesis is devoted to some unsolved theoretical aspects of optical manipulation. Since optical force acting on a micro-particle is typically on the order of pN and seldom larger than nN, it is a common belief that optical force is relevant in particle manipulation only when all other forces are comparable or smaller than the optical force. In chapter 2, surprisingly we showed that this is not always the case. Here, we find that under appropriate condition, optical vortices can make a sphere orbit around the beam center owing to the non-conservative optical force. If the sphere is attached to a mechanical spring, the spring can be stretched significantly even when the mechanical spring is orders of magnitude stronger than the optical force