A white rot fungus, Ceriporiopsis subvermispora, was shown to be able to depolymerize synthetic cis-1,4-polyisoprene (isoprene rubber; IR) in liquid medium, which was analyzed with size exclusion chromatography (SEC). During the 3-day incubation with C. subvermispora, the weight average molecular weight (Mw) of IR drastically decreased to be 24 % of those for starting IR. According to the SEC profiles, the decrease in Mw seems to occur via random scission of IR main chain. On the other hand, the change of Mw of IR shifted from decreasing to increasing during 3 to 7 days of incubation, and then the shift reversed during 7 to 28days. This phenomenon appears to be caused by cross-linking among main chains via radical coupling implying that unpaired electrons, radicals, happen on the main chain of IR. From these results, under incubation with C. subvermispora, it is supposed that IR is depolymerized and/or cross linked by free radicals via oxidative radical reaction like lipid peroxidation, i.e., carbon-centered radical (C·) happens on the IR chain leading it to chain scission to show reducing Mw, otherwise, two of C· on different IR chains couple to form cross linkage increasing their molecular weight.