Imaging and spectroscopic methods of two-dimensional laser-induced fluorescence (2D-LIF), ultraviolet rayleigh scattering (UV-RS) and re-decomposition laser-induced fluorescence (ReD-LIF) have been developed to observe the species in laser ablation process. These were applied to observed the laser ablation process of Si target, and it was shown that the atoms in ablation plume condense to molecules, clusters and finally to nanoparticles. The behavior of nanoparticles depended on by the viscosity of ambient gas. It was shown that these techniques are quite powerful to diagnose the condensation process in laser ablation.