N,N-dimethylhydroxylamine　(DMHA)　is　an　effective　novel　salt-free　reductant　used　in　spent　fuel　reprocessing.　This　paper　reports　the　effect　of　monomethylhydrazine　(MMH)　on　gamma-ray　radiolysis　and　radiolytic　by-products　of　DMHA　in　HNO3.　The　presence　of　MMH　improves　the　stability　of　DMHA　against　radiation　significantly,　the　main　radiolytic　by-products　are　N-methylhydroxylamine,　HCHO,　HCOOH　and　HNO2,　which　concentration　increases　with　the　dose.　The　concentration　of　N-methylhydroxylamine　increases　with　the　MMH　concentration,　but　that　of　HCHO,　HCOOH　and　HNO2　decreases　with　increasing　MMH　concentration.　These　results　will　provide　considerable　reference　for　the　application　of　DMHA　in　the　PUREX　process.