An IP fax and working method thereof, referring to the technical field of utilizing IP technique to fax. This invention includes a mail sending/receiving module, a session initialization protocol fax device, opposite user agents, further includes an IP fax server; a control order port of the mail sending/receiving module is connected with the IP fax server; after receiving by IP fax server, the IP fax server will send a feedback information to the mail sending/receiving module; the first input end of the IP fax server is connected with an input end of the session initialization protocol fax device so as to realize the inter-conversion flowing from T.38 to the fax file form; the second input end of the IP fax server is connected with an input end of the opposite user agents so as to realize the encapsulation to the session initialization protocol stack. The target of this invention is to provide a target of source file fax transmission which has distinct gradation, simple protocol, powerful function, supports multi-path fax session, fax file distributing mailbox and plural file forms.