Polystyrene, a polymer extensively used in the biomedical field, causes a problem for some applications because of its surface hydrophobicity. Nitrogen plasma could transform this shortage through polar group attachment. To understand the role of hydrogen during surface functionalization in the nitrogen cold plasma, the effects of the nitrogen and the mixture of N-2/H-2 plasma are investigated by both the examinations of the densities of attached amine groups and the in-situ diagnostic analyses such as optical emission spectroscopy and mass spectrometry. An increase of functionalization has been proved to be controlled by the gaseous NH radical formation when H-2 is added.