Maleic anhydride (MA) was grafted onto poly(β-hydroxybutyrate) (PHB) chains by using styrene (St) as co-monomer and dicumyl peroxide (DCP) as free-radical initiator. The grafting degree of MA was increased from 0.2 wt% to 0.9 wt% with the St/MA ratio up to 2/1. The St-assisted grafting mechanisms are revealed based on titration, thermogravimetric analysis and infrared analysis, i.e. MA is grafted onto PHB chains by way of single monomers and charge-transfer-complex (CTC) at the St/MA ratios of ≤1/1, while St-co-MA oligomers are the dominating grafting pendants when the St/MA ratios ≥2/1. The decomposition temperature (Td-5%) of PHB is raised by approx. 35 °C after grafting of the MA and/or short St/MA pendants, meanwhile higher molecular weight of the PHB is obtained. In addition, the crystallization of PHB is promoted by grafting of the MA or MA-co-St oligomers. The crystallization mechanism of maleated PHB is proposed as well.