Hydrothemal synthesis of gamma-Al2O3 nanoparticles obtained from aqueous Al(NO3)(3) solutions was carried out at 673 K and under pressures ranging from 30 to 40 MPa for 2 s using a flow apparatus. Effect of pressure and motality of HNO3 and KOH on crystal phase was examined. All products were a mixture of gamma-Al2O3 and AlOOH having crystallite sizes less than 18 nm. Weight ratio of gamma-Al2O3 to AlOOH in the products evaluated from TG showed a good correlation with pH at 673 K and increased from 0.17 to 1.16 with decreasing pH.