The molecular diversity of rumen methanogens in cattle fed tanninrich banana leaves or supplemented with commercial tannin extract was assessed using PCR-DGGE. Primer set 0357 F-GC and 0691 R was used to amplify the methanogenic archaeal community of the rumen. A total of 17 DNA fragments were excised from DGGE gels and their nucleotide sequences were determined. PCR-DGGE band profile and nucleotide sequence analysis revealed that methanogen resembling Methanobrevibacter thaueri strain CW and Methanobrevibacter millerae strain ZA-10 are the predominant methanogens in cattle. Furthermore, feeding of tannin-rich banana leaves to cattle significantly increased blood urea nitrogen but decreased rumen fluid ammonia nitrogen (RF NH3-N) level suggesting a protective effect of tannin on feed protein and possible inhibitory effect on methanogens by limiting the supply of available hydrogen from ammonia.