The atomic thickness of two-dimensional materials provides a unique opportunity to control their electrical(1) and optical(2) properties as well as to drive the electronic phase transitions(3) by electrostatic doping. The discovery of two-dimensional magnetic materials(4-10) has opened up the prospect of the electrical control of magnetism and the realization of new functional devices(11). A recent experiment based on the linear magneto-electric effect has demonstrated control of the magnetic order in bilayer CrI3 by electric fields(12). However, this approach is limited to non-centrosymmetric materials(11,13-16) magnetically biased near the antiferromagnet-ferromagnet transition. Here, we demonstrate control of the magnetic properties of both monolayer and bilayer CrI3 by electrostatic doping using CrI3-graphene vertical heterostructures. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened/weakened magnetic order with hole/electron doping. Remarkably, in bilayer CrI3, the electron doping above similar to 2.5 x 10(13) cm(-2) induces a transition from an antiferromagnetic to a ferromagnetic ground state in the absence of a magnetic field. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages.