Aggregation process of isotactic poly(methyl methacrylate)(i-PMMA)has been studied extensively for many years,and considerable progress has been made in both experimental and theoretical studies.Thev are,however,seldom sustained by real-space observations of the underlying morphology.In this paper,the aggregation process of i-PMMA in concentrated acetone solutions and the fractal structure of the resulting three-dimensional clusters were characterized on the basis of real-space AFM observations of their two-dimensional projection.It was found that spherical multiple-chain particles formed upon collapse and aggregation of the involving chains as a whole during quenching the solution to room temperature.By keeping the solution at room temperature,the initially formed particles stick together upon contact to form larger particles through reassembling very slowly.The succeeding collision of the enlarged spherical particles leads to the formation of small clusters.These newly formed small clusters grow when they meet with other clusters or single Brownian particles.This leads to the formation of large clusters with fractal dimension of 1.95±0.05.which suggest a reaction-limited cluster aggregation of i-PMMA in a concentrated acetone solution.This is in accordance with the conclusion obtained by light scattering measurements.