The oxidation behavior and the thermal stability of nanocrystalline Al-25Ti-8Mn intermetallic compound were investigated. $Al_3Ti$ intermetallic compound, which has a potential for high temperature structural material, was fabricated by mechanical alloying(MA) with $8at.\%$ Mn to enhance the thermal stability and ductility. And Al-25Ti-8Mn intermetallic compound was sintered by spark plasma sintering(SPS) at $700^{circ}C$. After sintering process, cubic $Ll_2$ structure was maintained without phase transformation and the grain size was about 50nm. To investigate the oxidation behavior of the specimens, thermal gravimetric analysis(TGA) was performed at 700, 800, 900, and $1000^{circ}C$ for 24 h in $O_2$. As the temperature increased from $700^{circ}C;to;900^{circ}C$ the weight gain of specimens increased. However at $1000^{circ}C$, unlike the oxidation behavior of $700^{circ}C;to;900^{circ}C$, the weight gain of specimen decreased drastically and the transition from linear rate region to parabolic rate region occurred rapidly due to the dense $alpha-Al_2O_3$.