The paper proposed a simple constitutive law and combined it with a Monte Carlo method to describe the evolution of size and fractal dimension of aggregates with different fractal dimensions in coagulation. Their formula for determining the fractal dimension of an aggregate in coagulation is formulated under a condition that the size of primary particles consisting of aggregate is kept constant in the course of coagulation process. This comment shows that a more general expression for determining the fractal dimension can be derived by the use of a variable size of primary particles and mass conservation law. (C) 2014 Elsevier Ltd. All rights reserved.