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Synchrophasor-based electromechanical modes extraction offers essential information for monitoring the small-signal stability of power systems without models. This article proposes an optimal mode decomposition (OMD) method for extracting electromechanical modes from noise-contaminated measurement data. In the proposed method, an iterative optimal process based on the Grassman manifold is employed to enhance robustness to measurement noise, such that the fixed low-dimensional subspace using proper orthogonal decomposition (POD) is not required in the proposed method. The proposed method gives a reliable extraction of frequency and damping ratio as well as the mode shape using multichannel wide-area measurement data. The performance of the proposed method is tested using simulated data from the IEEE 4-machine 2-area test system, 16-generator 5-area test system and measured data from a real power system.