In a human immune system, T cells play a significant role in defense against external invasion and have different functionalities depending on the phenotypes. Because these phenotypes are largely dependent on transcription factors that control gene expression, it is essential to know the change in TF in response to phenotype changing. In this study, we established a regulatory network for how transcription factors act on target genes within T cells. Based on this, we introduced the methodology for calculating the expression contribution during changing phenotype (ECCP), the indication of which transcription factor plays an important role in a particular differentiation process. This methodology considered gene expression according to T cell phenotype, chromatin accessibility changes, and the motif occurrence of transcription factor motifs in the regulatory sequences of target genes. This study has great significance in that it proposes the methodology that discovers important transcription factors by computational method and has enormous potential in that it can be applied to other cells groups other than T cells.