A novel method of ''sealed annealing'' was proposed to obtain large crystalline domains in organic deposited films. The deposited film of naphthalene tetracarboxylic derivative having long alkyl chains ( NTCDI-C8H) was annealed with a cover substrate put on it near the melting point. The cover substrate prevented melting or sublimation of the deposited film, resulting in a growth of large domains having a size reaching several hundreds microns, which can be clearly observed by a polarizing microscope.