Abstract: Gammaradiation decomposition effects were researched on O, O’-dimethyl methyl phosphonate (DMMP) as a stimulant for sarin. Gamma ray dose rate 40, 60, 77, 98, 115 Gy/min and dosage 100, 200, 500, 1000, 2000 kGy from a cobalt 60 gamma resource were used in this study. Irradiated samples were analyzed with gas chromatography and measurement errors of remaining DMMP were detailed with its influencing factors. They were shown that process of sealing and sampling led to DMMP loss about 0.33%-0.52%, DMMP contents of parallel samples differed by 0.08%-0.71%, standard deviation of contents value was about 0.01%-0.40%, and storage period also led to remarkable variance of DMMP content in irradiated samples which ranged from -1.8% to +2.9%. DMMP residue content inside samples were determined to decrease evidently with increase of dosage while indistinctively with increase of dose rate, which meant that the dosage should be a dominant factor. GC-MS analysis showed a number of decomposition products including gaseous small molecules and organic phosphorus molecules, which was in accord with free radical reaction mechanism. Radiation chemistry yields G were calculated within 0.75-1.0 μmol/J, which decreased with increase of dosage. With most intensive gamma ray parameters including dosage 2 000 kGy and dose rate 114.92 Gy/min, as much as 83.7% of DMMP was found undecomposed. Decomposition rate should be 16.24%, indicating a single mode of physical or chemical action unable to fully decompose DMMP, and also showing the need for combination of multiple means of degradation.