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Polystyrene (PS) with molecular weight of 2.58 × 10 was grafted to multi-walled carbon nanotubes (MWCNTs) via covalent bonds. The modified tubes can self-assemble as regular bundles in tetrahydrofuran (THF) and benzene while unmodified ones cannot but agglomerate seriously and irregularly in the same solvents. In order to find out the role that polymer plays in self-assembly, a series of PS in different molecular weights were synthesized and grafted on the surface of MWCNTs. THF and benzene are two selective solvents because they are well compatible with PS but bad with tubes. Consequently, grafted MWCNTs show amphiphilicity in them. Solvent-philic/solvent-phobic interaction controls the self-assembly driving force. Small PS offers MWCNTs worse compatibility and poorer amphiphilicity than big PS. With amphiphilicity faded, grafted tubes lose self-assembly ability gradually. Meanwhile, degenerated compatibility cripples tubes' mobility. Therefore, both number and size of self-assemblies (SAs) decline when molecular weight of grafted PS diminishes. SAs lose their regularity and structural integrativity at the same time. Turbiscan stability index was introduced to quantify the compatibility between dispersate and solvents. Besides, a solvent-philic/solvent-phobic balance system was proposed to calculate compatibility and amphiphilicity of all the modified tubes. This model establishes a connection between compatibility, amphiphilicity, and self-assembly behaviors of modified MWCNTs. [ABSTRACT FROM AUTHOR]