The goal of this thesis is to develop a scientific and engineering basis needed for production of thermoplastic foams through clarifying the fundamental foaming mechanisms blown with a chemical blowing agent (CBA) under the atmospheric pressure. The presented research is based on a series of visualization experiments by using an optical microscope and a CCD camera system, in which foamable polyethylene (PE) and polypropylene (PP) samples were used. Samples prepared by the compression molding method were employed to clarify the dynamics of bubble nucleation, growth and verify a mathematical bubble growth model about lifespan of CBA-blown bubbles in non-pressurized polymer melt, and samples prepared by the master batch method were used to investigate the influencing factors on foam morphology in rotational foam molding. The decomposition behaviors of the selected CBAs were analyzed by conducting thermal analysis experiments. The mechanisms that govern the cell nucleation, cell growth and shrinkage were clarified. The critical processing parameters that would affect the development of a fine-celled foam structure in rotational foam molding were also outlined.