In this study, two pilot-scale pig manure composting piles were constructed for a systematic investigation of arsenic (As) methylation during manure composting. Microbial community composition, as well as the abundance and diversity of arsM genes were monitored using real-time PCR (qPCR) and amplicon sequencing of both 16S rRNA and arsM genes. Results show an overall accumulation of methylated As occurring during 60 day-composting time. The arsM gene copies increased gradually over time and were correlated positively to the concentrations of methylated As. 16S rRNA gene sequencing and arsM clone library analysis confirmed that the high abundance and diversity of arsM genes shared the same known As-methylating microbes, including Streptomyces sp.,Amycolatopsis mediterranei and Sphaerobacter thermophiles, which were likely involved in the methylation process. These results demonstrated that As methylation during manure composting is significant. For the first time, the linkage between As biomethylation and the abundance and diversity of the arsM functional gene in composting manure was established.