A modular heater system includes a plurality of modular units aligned along a longitudinal direction of a fluid conduit for heating a fluid contained in the fluid conduit. The modular units each include a carrier member, a heating element mounted on the carrier member and a thermal insulation jacket surrounding the carrier member and the heating element. The carrier member defines a receiving space for receiving the fluid conduit therein and includes side slots recessed from outer surfaces of the carrier member. The thermal insulation jacket includes an upper half portion and a lower half portion. The upper half portion and the lower half portion are self-locked in the side slots of the carrier member.