The growth kinetics of nanograins in NiO nanofibers has been investigated. Individual NiO nanofiebrs synthesized by electrospinning consist of nanograins. The nanograins coalesce and grow at the expense of smaller ones under higher calcination temperatures and prolonged calcination times. The growth kinetics of nanograins makes a transition during growth at a critical size of similar to 30 nm in diameter. At the early stage of growth, the activation energy and the growth exponent are found to be 10.94 kJ mol(-1) and 9.09, respectively. They change to 137.58 kJ mol(-1) and 2.04 at the later stage of growth at the critical size of nanogrians. In that growth stage, bamboo-like grains start to evolve. (C) 2011 Elsevier B.V. All rights reserved.