The transcription initiation of eukaryotic class Ⅱ genes is very complicated and strictly regulated. Many events such as the alteration of chromatin structure, the interaction between cis-elements and trans-factors are involved in this process. Based on the results of in vitro reconstitution transcription and the discovery of RNAPⅡ holoenzyme in yeast respectively, two models, step-wise assembly model and RNAPⅡ holoenzyme model, were proposed on the assembly of basal transcription initiation complex at promoters. Two related processes, antirepression and true activation, were supposed to be the molecular mechanism of the transcriptional activation.