Castro called Jose Miguel Insulza an "idiot" for "yielding" to U.S. pressure.
Last week Insulza called for the advancement of democracy and protection of human rights in Cuba during the "Community of the Democracies" conference in Santiago, Chile.
The new OAS head -- Chile's former interior minister -- denounced Castro's remarks and refused to elaborate.
Insulza said last week that leaders in the region are often divided on the issue of Cuba. The Organization of the American states suspended the membership of Cuba in 1962 after Fidel Castro came to power.
"Everyone (at the OAS) wants to advance democracy and human rights in Cuba," said Insulza. "We are working in that direction."
Chile's President Ricardo Lagos also came to the defense of Insulza and said Cuba must be responsible for its own efforts at intergration into the OAS.