This paper investigates the effects of LPG composition, Which is injected during air inlet period, on emissions and performance characteristics. The engine has modified to determine the best LPG composition for dual operation in order to improve the emissions quality while maintaining high thermal efficiency in comparison to a conventional diesel engine, a single cylinder, naturally aspirated, four-stroke, DI diesel engine converted to run as dual fuel engine. LPG injection rate were selected as 5%, 10%, 15% and 25% on a mass base. It has been that although maximum increase in brake power and brake torque were found as 21.59% and NOx emission decreased by 44.7% at 1200 rpm with 25% LPG, brake specific consumption (SFC) and smoke emission drastically increased by 16.7% and 80%, respectively. Minimum SFC and maximum brake efficiency obtained with 15%LPG between 1400 and 1800 rpm engine speeds. Optimum injection rate is found at 5% LPG in terms of exhausts emissions and performance. At this injection rate SFC, NOx and smoke emissions decreased by 9%, 27.6% and 20% at 1600 rpm, respectively