A novel "gradient" polymer, prepared by complex-radical terpolymerization of styrene (St), maleic anhydride (MA), and N-vinylpyrrolidone (NVP), was synthesized by use of a reversible addition-fragmentation chain transfer (RAFT) process. The terpolymerization was performed in tetrahydrofuran (THF) at room temperature under gamma-ray irradiation in the presence of dibenzyl trithiocarbonate (DBTTC). The constants of complex-radical terpolymerization, complex formation, and some kinetic parameters for the monomer systems were studied by UV, H-1 NMR, Kelen-Tudos methods, respectively. The equilibrium constant (K-2) of 1:1 complex between MA and NVP was determined by UV method as 3.57 x 10(-2) L/mol at 293.2 K in CHCl3. Obtained results show that the terpolymerization proceeded mainly through "complex" mechanisms in the state of near-binary copolymerization of St center dot center dot center dot MA (CTC1) and MA center dot center dot center dot NVP (CTC2) complexes. And as the big difference in the value of the reactivity ratios between the two complexes (r(1) = 0.17 +/- 0.01 and r(2) = 4.20 +/- 0.34), the terpolymer displayed the characteristics of "gradient" distribution with controlled molecular weight and narrow molecular weight distribution.