||The behaviors of colloidal particles at liquid-vapor interface are studied experimentally. Particles are found to aggregate towards the center of the curved liquid surface, due to buoyancy force. Two macroscopically distinct states, the triangular lattice and the packed Rod State of stacking particles, are observed in the aggregation process. It is also observed that there is a transition between the two distinct states. A simple model using buoyancy-induced pressure field explains the phenomena in this transition. A double layer construction is introduced to explain the packed Rod State. The order in 2D colloidal system under magnetic dipole interaction is also studied. A local order parameter S is introduced to characterize the system state. I find a logarithmic relation between S and the inner pressure. As to high-pressure approximation, I show theoretically that S is inversely proportional to the potential of particle site. Besides local order, the translational and orientational correlation is also studied. I show that the phase observed in the experiment is an isotropic liquid state, and the correlation length of the two correlation functions is consistent with the prediction of KTHNY theory .