The ignition delay of the pilot fuel in a dual fuel engine is different from that in a diesel engine because the primary fuel alters the properties of the charge, reduces the oxygen available and undergoes pre-ignition reactions during compression. In the present work, a correlation for the ignition delay in a biogas-diesel dual fuel engine has been proposed on the basis of experimental results. This correlation can be used if relevant parameters corresponding to diesel mode of operation and the properties of the gaseous fuel along with its concentration in the intake charge are known. The Hardenberg & Hase correlation for ignition delay in diesel engines has been modified for the dual fuel situation by bringing in to effect the changes in the temperature at the end of compression and oxygen concentration in the charge. The proposed correlation shows reasonable agreement with experimental results for a biogas-diesel dual fuel engine.