A hydraulic system of a forklift truck is provided to smoothly operate a lift cylinder by supplying hydraulic fluid of different sources to the lift cylinder as occasion demands. A hydraulic system of a forklift truck includes first and second hydraulic pumps(60,62), first and second sensing units(80,82), a bypass valve(87), and a control unit(85). The first hydraulic pump supplies hydraulic fluid to a lift cylinder(8) and a steering unit(52) through a first hydraulic line(64). The second hydraulic pump supplies the hydraulic fluid to a tilt cylinder(9) and various kinds of attachments through a second hydraulic line(66). The first sensing unit senses operation of the lift cylinder. The second sensing unit senses operation of the tilt cylinder and the attachments. The bypass valve is installed between the first and second hydraulic lines and selectively connects the first and second hydraulic lines. The control unit controls the bypass valve so as to supply the hydraulic fluid to the first hydraulic line in case of operation of the lift cylinder.
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