To investigate the growth mechanism of carbon nanofibers (CNF) through a process using the thermal decomposition of poly(ethylene glycol) (PEG), we researched the phenomena that occurred from PEG decomposition to CNF growth during a temperature elevation process of CNF synthesis. In the PEG thermal decomposition method, the selective synthesis of platelet CNF (PCNF) and cup-stacked CNF was accomplished by changing only the ramp rate (without any other difference in experimental conditions). As a result, it was confirmed that carbon-containing gases generated by PEG thermal decomposition were converted to amorphous carbon and deposited on the substrate. There was a clear correlation between the amount of deposited amorphous carbon and the size of the nickel catalyst particle for CNF growth. Therefore, in the PEG thermal decomposition method, amorphous carbon deposition was found to be important to control the dispersity and morphology of the catalyst particle, and it was probably crucial to the determination of the type of CNF. In addition, the PCNF prepared in this study showed multidirectional growth from polyhedral catalyst particles, and this could be why the PEG thermal decomposition was able to create PCNF with small diameters compared to PCNF synthesized by conventional methods.