This article designs a class of low-profile compact multiple-input-multiple-output (MIMO) antennas on a single radiating patch using simple and systematic characteristic modes (CMs) method. Rather than using conventional method of radiation pattern and polarization diversities, this work demonstrates that by taking advantage of a proper superposition of some even/odd modes, it is able to generate similar radiation pattern and polarization no matter which port is excited. It is also found that port isolation and impedance matching are dependent on not only operating modes but also input couplings. The proposed CMs method consists of three main steps. First, adjust desired even and odd modes to be equal in resonant frequency (ResF). Second, suppress unwanted modes without affecting desired ones. Lastly, determine feeding positions to obtain proper input couplings. In this way, satisfactory port isolation and impedance matching could be achieved simultaneously and quickly, getting rid of complicated decoupling structures and time-consuming optimization. To exemplify how the proposed CMs method works, two MIMO antennas are successfully designed by just using simple slots and shorting pins. A prototype for the proposed four-port MIMO antenna is fabricated and measured for validation.