The phase forming and development of polystyrene／poly(cis-butadiene) rubber (weight ratios were 70／30 and 50／50

respectively) blends during mixing were investigated by pattern analysis of scanning electron microscope．For the blend with composition 70／30

the average diameter(PDav) of the dispersed phase[particles(PeBR)] decreased in the initial stage of mixing

and fluctuated in the metaphaze and later stage．The distribution of the particles trended to uniform with the blending process．But for the blend with composition 50／50

which is near the phase inversion composition

the calculation of PDav is difficult because of the co-continuous structure of the two components

thus two parameters

the characteristic length A and the average characteristic length Am are used to characterize the size of the dispersed phase．The characteristic length was defined es the measuring distance of black area(dispersedphase) when scanned the binarized SEM images along a direction．The changes of m was similar to that ofPDav andthe A and Amcould reflectthe size and size distribution of the dispersed phase whether or not the composition is near the phase inversion．Furthermore

the histograms of Am obtained at various time in the metaphase and later stage of mixing fell on a master curve．demonstrating the self-similar growth of the phase structure．Thus a scale function P(A/Am) was defined to study the fractal behavior of the mixing．The results showed that the fractal dimension D could be used to characterize the mixing process of the polymer blend．