The important intermediate product of biosynthesis of fatty acid is

Malonyl CoA  Acetyl-CoA is the immediate substrate for fatty acid synthesis. However, acetyl-CoA needs to be converted to the activated form, which will serve as the donor of carbon units to the growing fatty acid chain. Malonyl-CoA is such activated form; acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase.