TFile *file = new TFile("filename.root", "RECREATE");
TTree *tree = new TTree("tree_name", "Title");
TClonesArray *myarray = new TClonesArray("NameOfYourClass");
tree->Branch("mybranch", "TClonesArray", &myarray);

// Event loop
for(Int_t i = 0; i < 100; i++)
{
    // Data loop
    for(Int_t k = 0; k < 20; k++)
    {
        new ((*myarray)[k]) ConstructorOfYourClass(..parameters..);
    }

    tree->Fill();
    myarray->Clear();
}
tree->Write();
file->Close();