Abstract: The high temperature gas-cooled reactor was structured by many graphite blocks and carbon bricks. Gaps between these graphite blocks were possible bypass flow paths of the reactor core. It is necessary to carry out detailed study on the bypass flows because the flow and temperature distributions in the reactor core may be affected. The vertical gaps between those side graphite reflectors were the main bypass flow paths, where helium might flow from the cold plenum into the hot plenum directly and mix with helium flows from the pebble bed in the transvers direction. Based on the flow network model for flows in the pebble bed and vertical gaps, the effect of transvers flow on the bypass flow was analyzed for various gap sizes in various positions. The results show that the transvers mixing of helium flow around the vertical gaps is not negligible to the bypass flow. Gaps in larger size are more important to the bypass flow. And the bypass flow ratio is larger with larger gaps.