A power relay assembly comprising: an upper case in which at least one electric element (10, 20, 30) is mounted on one surface thereof; a lower case (120, 220) coupled to the upper case; and at least one bus bar electrically connected to the electric element (10, 20, 30), disposed between the upper case and the lower case, and including a bottom portion in surface contact with at least one of the upper case and the lower case (120, 220), wherein at least one surface of the bottom portion is in contact with a portion formed of a plastic material having a heat dissipation property and an insulation property between the upper case and the lower case, wherein the upper case and the lower case (120, 220) are formed of a plastic material having a heat dissipation property and an insulation property, wherein the plastic material of the upper case and the lower case (120, 220) has insulating heat dissipation fillers dispersed on a polymer matrix, wherein the bottom portion is formed in a plate shape having a predetermined area, wherein the inner surface of the upper case in surface contact with the bottom portion is a protruding portion configured to protrude by a predetermined height, and whereby heat being generated when the at least one electric element (10, 20, 30) is operated is released after being transferred to both sides of the upper case and the lower case (120, 220), characterized in that one surface of the bottom portion is in surface contact with an inner surface of the upper case and one surface of the bottom portion is in surface contact with an inner surface of the lower case (120, 220). an upper case in which at least one electric element (10, 20, 30) is mounted on one surface thereof; a lower case (120, 220) coupled to the upper case; and at least one bus bar electrically connected to the electric element (10, 20, 30), disposed between the upper case and the lower case, and including a bottom portion in surface contact with at least one of the upper case and the lower case (120, 220), wherein at least one surface of the bottom portion is in contact with a portion formed of a plastic material having a heat dissipation property and an insulation property between the upper case and the lower case, wherein the upper case and the lower case (120, 220) are formed of a plastic material having a heat dissipation property and an insulation property, wherein the plastic material of the upper case and the lower case (120, 220) has insulating heat dissipation fillers dispersed on a polymer matrix, wherein the bottom portion is formed in a plate shape having a predetermined area, wherein the inner surface of the upper case in surface contact with the bottom portion is a protruding portion configured to protrude by a predetermined height, and whereby heat being generated when the at least one electric element (10, 20, 30) is operated is released after being transferred to both sides of the upper case and the lower case (120, 220), characterized in that one surface of the bottom portion is in surface contact with an inner surface of the upper case and one surface of the bottom portion is in surface contact with an inner surface of the lower case (120, 220).