An HVAC system of a vehicle includes: a battery line configured to thermally interconnect a first radiator and a high-voltage battery core and provided with a first pump; a refrigerant line having a compressor, a condenser, and an evaporator; an indoor cooling line interconnecting an indoor HVAC cooling core and the evaporator and having a second pump; an indoor heating line thermally interconnecting an indoor HVAC heating core and a condenser and having a third pump; battery cooling lines branching from opposite side points of the high-voltage battery core in the battery line, respectively, and connected to the indoor cooling line; battery heating lines branching from the opposite side points of the high-voltage battery core in the battery line, respectively, and connected to the indoor heating line; and a first valve disposed at one of the opposite branching points in the battery line.