Detailed laboratory investigations were conducted on Mae Moh fly ash from northern Thailand for the determination of its grain size distribution, mineralogy, pozzolanic activity, compaction and strength characteristics, and the collapse potential. On the basis of the experimental results, this fly ash is classified as ASTM class C, which is considered to be pozzolanic. It has good potential to be utilized as an effective fill for embankments (roads and dams), airfields, pavements, and building bricks, as well as for the stabilization of compressible or erodible foundations. Because of the fact that Mae Moh fly ash contains only a negligible amount of unburned carbon, its pozzolanic reactivity is accelerated, in comparison with the relatively inert, high-carbon fly ash produced elsewhere in Thailand and many other parts of Asia. It is also demonstrated that Mae Moh fly ash can be easily compacted to produce acceptable dry densities over a wide range of water contents. Curing with an adequate moisture supply in the presence of calcium oxide plays an important role in accelerating the pozzolanic reactions, hence improving the time-dependent-properties. This study further proposes that a curing period of 2–3 weeks is sufficient for this material to approach its maximum strength. Although the behaviour of one specific fly ash cannot generalize the wide array of other ashes, the test results obtained for Mae Moh fly ash may be applied to lignite ashes in the category of ASTM class C. Key words: fly ash, structural fill, compaction, compressive strength, shear strength, collapse potential, pozzolanic activity.