An interactive whiteboard system and a whiteboard writing instrument thereof are provided. The interactive whiteboard system comprises a whiteboard, a whiteboard writing instrument, a detection device and a projection device. The whiteboard writing instrument comprises a housing and an internal element that has a writing terminal. The writing terminal receives a pressure from the whiteboard when a user holds the whiteboard writing instrument to write contents on the whiteboard. The housing presents a pattern, which experiences a variation according to the pressure so that the detection device detects the variation of the pattern to get information corresponding to the contents. The projection device projects an image corresponding to the contents onto the whiteboard according to the information.
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