This report proposes a new controlled islanding strategy for power grids based on the Koopman Mode Analysis (KMA). Controlled islanding is a strategy of emergency control of a power grid, in which the grid is intentionally split into a set of islanded sub-grids for avoiding the entire collapse. The KMA is a new technique of nonlinear modal decomposition based on properties of the point spectrum of the so-called Koopman operator. A key point of the proposed strategy is that we determine the set of islanded sub-grids using KMA of data on voltage angle dynamics of every bus. The strategy is numerically investigated with the IEEE 68-bus test system. By simulations of controlled islanding in the test system, it is demonstrated that the grid's response following a fault is improved with the proposed strategy.