A series of polypropylene-graft-polystyrene (PP-g-PS) were prepared by solid-state graft polymerisation of styrene (St) in the presence of tert-butyl perbenzoate (TBPB) and 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO) using nascent spherical isotactic polypropylene (PP) granules as matrix. Reaction conditions such as ratio of TEMPO to TBPB, amount of TBPB and St, reaction time and temperature were investigated. The grafting degree (GD) decreased slightly with the addition of TEMPO, but the introduction of TEMPO reduced gel formation and degradation of PP chain. Introduction of camphor sulfonic acid in the TEMPO-mediated grafting polymerization system lead to evident increase in molecular weight of PP-g-PS and decrease of gel formation with slight changes of GD. GPC curves of the product synthesized with TEMPO addition showed obvious tailing at high molecular weight end. Complex viscosity In dependence on angular frequency of the melt of product showed obvious shear-thinning behaviors. All the experimental results indicated existence of long-chain branches in the product of graft polymerization, (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112: 275-282, 2009