Well-crystallized manganese ferrite nanoparticles with mean size of 20 nm were synthesized at low temperature (<= 100 degrees C) by ultrasound wave-assisted solid-liquid reaction ball milling technique without subsequent calcination. MnO2 were used as the raw material and iron milling balls with diameter of 1.0-1.5 mm were used. As a comparison, solid-liquid reaction ball milling without ultrasonic wave assistance and the effects of different milling balls were also investigated. The results showed that, this technique is simple, environmentally friendly and energy-saving for nanocrystal synthesis. (c) 2012 Elsevier B.V. All rights reserved.