Gelation dynamics and gel structure of fibrinogen induced by serine protease, thrombin, was investigated by light scattering, real space observation using confocal laser scanning microscopy (CLSM), and turbidity. Effects of additives, such as (linear) saccharides, glucose to dextran, and cyclodextrin, were studied focusing on the interaction with fibrin(ogen) and thrombin. Light scattering measurement was ascertained to be able to characterize the gelation process and growth kinetics. Stepwise (two-step) gelation process, formation of fibrin monomers and protofibrils followed by the lateral aggregation to form fibrin fibers and gel network, was clearly ascertained. Gelation point could be characterized quantitatively. At the gelation point, dynamic light scattering exhibited a self-similar nature of the fibrin gel network, and the fractal dimension was evaluated in good accordance with the reconstructed 3D image of gel network by CLSM. The interaction between the additives and fibrin(ogen) and thrombin were studied by the inhibition test using synthesized substrate. Temporal variation of microstructure of fibrin gel network (lateral fiber growth) was investigated by turbidity in detail. Addition of saccharides affects significantly the network formation as revealed by turbidity. The interaction of dextran with fibrin fibers was examined by fluorescence microscopy, too, and the characteristic spatial distribution was observed.