Abstract: Three-dimensional reconstruction of composite from serial sections has become an important way to obtain microstructure information such as volume fraction, size, shape and spatial distribution of the reinforcements.Nevertheless, the 2D sectional contour of each second-phase particle obtained by the active contour model is often unfair.This paper presents a novel method to smooth the particle contour with cubic B-spline curve approximation.In particular, the knots of a parametric B-spline curve are treated as variables, and the optimized knot placement is accomplished by considering the locations of knots as genes;the PBIL algorithm is then applied to attain the best resolution.The common knot vector of the serial contours of the same particle is constructed by inserting knots based on flexibility interval.Finally, a bicubic closed B-spline surface with C2 continuity is built to represent the particle surface.Experimental results show that the proposed method is robust, with better precision than that by the genetic algorithm based knot placement hence satisfactory reconstructed shape of the particles.