The present work attempts to unfold the effect of compression ratio on performance, combustion and emission characteristics of a rice bran biodiesel-biogas run dual fuel diesel engine. For experimentation, a single cylinder, direct injection (DI), natural aspirated (NA), water cooled, variable compression ratio (VCR) diesel engine is converted into a biogas run dual fuel diesel engine. Experiments were conducted at three different compression ratios (CRs) of 18, 17.5 and 17, and at a fixed injection timing of 23 bottom top dead centre (BTDC) under different loading conditions. At full load, the maximum brake thermal efficiencieS (BTEs) under dual fuel mode (DFM) are found to be 20.27%, 19.97% and 18.39% for CRs of 18, 17.5 and 17, respectively. For same loading condition, the maximum liquid fuel replacement (LFR) is found to be 80%, 79% and 78.2% for CRs of 18, 17.5 and 17, respectively. On an average, there is a reduction in carbon monoxide and hydrocarbon emissions by 17.67% and 17.18% when CR is increased from 17 to 18. However, for the same setting of CRs, there is an increase of oxides of nitrogen as well as carbon dioxide emissions by 42.85% and 14.13%, respectively. (C) 2015 Elsevier Ltd. All rights reserved.