Strain 39 is an endophytic fungus which was isolated from Dioscorea nipponica Makino (DNM). After Strain 39 co-cultured with ethanol extract of DNM rhizomes for several days, the content of saponins in this culture mixture would be obviously increased. To analyze the mechanism of this microbial transformation, we used the differential display reverse transcription polymerase chain reaction (DDRT-PCR) method to compare the transcriptomes between Strain 39 cultured in normal PD medium and in PD medium which added ethanol extract of DNM rhizomes. We amplified 29 DDRT-PCR bands using 12 primer combinations of three anchored primers and five random primers, and six bands were re-amplified. Analysis of real-time PCR and sequence alignment showed that three clones were up-regulated in sample group: squalene epoxidase, squalene synthase, and catalase, one clone was expressed only in sample group. The possible roles and origins of the above genes were discussed, and the molecular mechanism of Strain 39 biotransformation was speculated. This study is the first report of the molecular biotransformation mechanism of saponins production by endophytic fungus of DNM.