In this paper we present the morphological characterization of palladium (Pd) nanoclusters obtained by low-pressure plasma sputtering and dedicated to catalytic carbon nanotube growth. It is shown that small Pd clusters deposited on silicon are not stable at the processing temperature (873 K); they tend to migrate and coalesce with their close neighbors. Heat treatment in fact leads to a bimodal cluster size distribution (2 nm and 10-30 nm), starting from 5-nm as-deposited nanoclusters. Another consequence is the appearance on the silicon substrate of areas containing low (isolated aggregates) or high cluster density. Knowledge of the metal cluster thermal evolution is of great importance in understanding the nanotube growth mechanism. First results indicate that the carbon structures grow only on particles of large size.