The bio-fuel, 2,5 - dimethylfuran (DMF) is currently regarded as a potential alternative fuel to gasoline due to the development of new production technology. However, little is known about the flame behavior in an optical engine. In this paper, high speed imaging (with intensifier) was used during the combustion of DMF and its blends with gasoline and ethanol (D50, D85, E50D50 and E85D15) in an SI optical engine. The flame images from the combustion of each fuel were analyzed at two engine loads: 3bar and 4bar IMEP. For DMF, D50 and E50D50, two modes were compared: DI and PFI. The average flame shapes (in 2D) and the average flame speeds were calculated and combined with mass fraction burned (MFB) data. The results show that when using DMF, the rate of flame growth development and flame speed is higher than when using gasoline. The differences in flame speed between DMF and gasoline is about 10% to 14% at low IMEP. Also the flame propagation when using DMF is similar to that when using ethanol. The flame propagation rates and flame speeds of the blended fuels reside between those of the pure fuels. The behavior of combustion is more highly influenced by the addition of DMF than when using either gasoline or ethanol. The results of the flame propagation process and flame speed prove that DMF's existence in the fuel blends will bring the combustion closer to that with pure DMF, even when using low percentages. The flame speed when using DMF in DI is significantly higher than in PFI mode at MFB5. For D50 and E50D50, the flame area in DI is always bigger at the same crank angle. The inhomogeneous mixture formed by DI affects the combustion and flame speed of DMF significantly.