We have used STM to examine the effects of annealing time in the presence of different gaseous species (Ar, H-2 and N2) on the morphological transformation of vacuum-vapor deposited Pt clusters on highly oriented pyrolytic graphite (HOPG). This study shows that the morphological transformation of the Pt clusters on HOPG is a function of both gaseous species and annealing time. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealing for 4 h in the presence of Ar and H-2. Further annealing shows a morphological transformation from spherical to elongated shape. Small amounts of hydrocarbon impurities in Ar or hydrogen might possibly explain the shape transformation of the Pt clusters. However, the morphological transformation of the Pt clusters is totally different when the sample is annealed in low sticking coefficient gas, N2, suggesting adsorbed gases (hydrocarbon or hydrogen) have profound effects on the shape transformation. These results indicate that pretreatment conditions are important factors in the shape transformation, which may have significant influence on catalytic activity and selectivity.