Alkyl methacrylate-maleic anhydride (RMC-MA) copolymers are among the most widely used pour point depressants (PPD). In order to develop more efficient RMC-MA copolymers, it is necessary to study the crystallization behavior of n-alkanes when adding RMC-MA into diesel fuel. In this paper, RMC-MA is prepared by the reaction of long-chain alkyl methacrylate and maleic anhydride. The diesel fuel before and after adding C14MC-MA is in situ filtered at its cold filter plugging point (CFPP) in a manual CFPP apparatus. Extensive measurements of composition variations of n-alkanes are conducted by gas chromatography and the results are compared. The experimental results show that after adding C14MC-MA, the concentration distribution of n-alkanes in the filtrate is wide and ranges from 8 to 28 carbon atoms, mainly centralizing from 10 to 19 carbon atoms. For the precipitate, the concentration distribution of n-alkanes gets richer in the lighter n-alkanes and poorer in the heavier n-alkanes. The concentration distribution of n-alkanes in the crystal solid shows a decreasing trend, especially with high carbon number n-alkanes (heavier than 20 carbon atoms). About 60% of the residual crystal solid is composed of nonparaffins such as isoparaffin, naphthene, and other components. Crystallinities of n-alkanes show a tiny decreasing trend from C-8 to C-20. When the carbon number of the n-alkanes is more than C-20, the crystallinities of n-alkanes begin to sharply reduce with an increase of carbon number. The largest decline of crystallinity is C-26 n-alkane from 38.4% to 3.4%.