Spin physics in nanometer scale is a topic of importance in modern physical research. Nitrogen Vacancy centers (NV centers) are point defects with atomic dimension in the diamond lattice. The orbital ground state of NV centers is a spin triplet whose spin state can be optically polarized and read out. Due to its long coherence time and atomic size, single NV centers in diamond are ideal systems for implementing spin physics experiments. In this thesis, two aspects of spin physics, the Berry phase of a spin in time-dependent magnetic field, and detection of external spins are explored with single NV centers in diamond. In our Berry phase experiment, the Berry phase is accumulated and measured with NV center and the decay of signal is discussed in details. The Berry phase experiment provides better understanding of the essence of geometric phase. The idea of using the robustness of geometric phase against certain types of errors to circumvent the bottleneck of lacking accurate control of dynamic phase is critically discussed. The detection of external spins with NV center is realized with double electron-electron resonance (DEER) technique, by driving the target spin actively with pulsed microwave radiation. This result opens the gate to a variety of applications of NV center magnetometry in interdisciplinary topics such as researches on dynamics of protein molecules with electronic spin labeling.