An attempt was made to clarify the effect of polymerization conditions (e.g., the content of chain transfer reagent, irradiation dose, acrylonitrile (AN)–urea ratio, aging time) on the stereoregularity, the viscosity-average molecular weight Mv, and the conversion in the radiation-induced polymerization of AN–urea canal complex in the presence of radical chain transfer reagent including alkyl mercaptans, alkylamines, alkyl alcohols and chloroform. The addition of n-butyl mercaptan (n-BM) to AN–urea system slowed down the rate of the canal complex formation and decreased the amount of the canal complexes formed. The role of n-BM on the formation of AN–urea canal complex was also examined by DSC. The enthalpy of fusion (ΔH) of AN solid in AN–n-BM–urea system decreased with elapse of aging time, although ΔH of n-BM was almost constant in the whole range of aging time. From these findings, it was concluded that n-BM is not included in the AN–urea canal complex. The conversion decreased remarkably with an increase in n-BM content. When 2 mol%/AN of n-BM was added to AN, Mv was almost constant (ca. Mv=2.0×105), irrespective of the amount of n-BM added. Addition of n-BM to AN–urea system brings about a small, but significant increase in the content of isotactic triad (mm) by 5 to 14%.