Provided is a positive electrode active material for lithium ion batteries, which is capable of realizing stability and safety at a high voltage, a high energy density, high load characteristics, and long-term cycle characteristics by controlling a crystal shape of LiMnPO4 particles having a crystal structure very suitable for Li diffusion or controlling an average primary particle size, a production method thereof, an electrode for lithium ion batteries, and a lithium ion battery. The positive electrode active material for lithium ion batteries of the invention is a positive electrode active material for lithium ion batteries, which is formed from LiMnPO4. Values of lattice constants a, b, and c, which are calculated from an X-ray diffraction pattern, satisfy 10.41 Å