We have developed the Epi-Base SATURN process as a silicon bipolar process technology which can be applied to optical transmission LSIs. This process technology, to which low temperature selective epitaxial growth technology is applied, is based on the SATURN process. By performing selective epitaxial growth for base formation in 2 steps, transistors with a 40GHz maximum cut-off frequency have been fabricated. In circuit simulation based on SPICE parameters of transistors, the target performance required for 2.4 Gbit/s optical interface LSIs has been achieved.