<p>REDD+ mechanism emphasizes mainly on carbon stock enhancement but biodiversity as co-benefit. Participatory managed forest has been preparing to get benefit under this mechanism and collaborative forest is one of them in Nepal. Research objective is to assess the relationship between C stock and biodiversity. Three collaborative forests (CFMs) were selected for study site in Mahottari district, Nepal. Altogether 96 sample plots were established applying stratified random sampling. The plot size 20mx25m for tree was established and other concentric plots were established. Diameter at breast height (DBH) and height were measured, species were counted and soil samples were collected specifically from 0-0.1, 0.1-0.3 and 0.3-0.6m depths. The biomass was calculated using equation of Chave et al and converted into carbon, soil carbon was analyzed in laboratory and plant biodiversity was calculated. Then, relation between C stock and bio-diversity was evaluated. Estimated total carbon stock was found 1090593.00 t in CFMs. Out of this, there were 395398, 296927 and 398268 t C stocks in Banke- Maraha, Tuteshwarnath and Gadhanta- Bardibash CFMs respectively. The values of Shannon-Weiner Biodiversity Index ranged 2.21-2.33 and no any significant relationship was existed between carbon stock and biodiversity. Hence, REDD+ mechanism should emphasize on biodiversity conservation.</p>