An ion implanter and an ion implant method are disclosed. The ion implanter has an aperture assembly with a variable aperture and is located between an ion source of an ion beam and a holder for holding a wafer. At least one of the size and the shape of the variable aperture is adjustable. The ion beam may be flexibly shaped by the variable aperture, so that the practical implantation on the wafer can be controllably adjusted without modifying an operation of both the ion source and mass analyzer or applying a magnetic field to modify the ion beam. An example of the aperture assembly has two plates, each having an opening formed on its edge such that a variable aperture is formed by a combination of these openings. By respectively moving the plates, the size and the shape of the variable aperture can be changed.