In bacterial DNA replication, DnaG primase synthesizes RNA primers which are then extended by DNA polymerase. The DnaG primase consists of three domains, N-terminal zinc-binding domain (ZBD), RNA polymerase domain (RPD) and C-terminal helicase binding domain (HBD). In the process of producing primers, the three domains of primase cooperate with each other, and none is dispensable. Although the structures of the primase domains have been reported, so far, the full-length structure of the primase is not known yet. Here, the model of full-length DnaG in Bacillus subtilis (BsuDnaG) was constructed from the data of X-ray small angle scattering (SAXS) analysis. The BsuDnaG is in extended state in solution. On the other hand, the ZBD and HBD domains could exhibit continuous conformational changes relative to the RPD domain. This study suggests the domains rearrangement in DnaG primase may facilitate its function in DNA replication.