A sheet type balloon has a blank, colorable area partly formed on an outer surface of a body, so the balloon is available for coloring or studying materials as well as typical playing materials. The balloon further has a valve assembly having bi-directional functions, i.e., deflation and inflation of the balloon. To inflate the balloon with air or gas, a fastener of the valve assembly is open by separating a convexly protruding line and a concavely protruding line. After inflation, the fastener is closed by apply a pressure to both protruding lines, so the protruding lines are closely combined and thus intercept the outward flow of inflating air or gas within the body. Then, for carrying or reuse, the fastener can be open again by separating the protruding lines.