A method of constructing a circular template includes preparing a partially double stranded linear DNA molecule, and incubating the partially double stranded linear DNA molecule with a ligase capable of intra-molecular ligation of single stranded DNA molecules to generate a partially double stranded circular DNA molecule. A method of detecting DNA molecules includes the following steps. Target DNA molecules are isolated with probes to form partially double stranded linear DNA molecules. The partially double stranded linear DNA molecules are incubated with ligases capable of intra-molecular ligation of single stranded DNA molecules to generate partially double stranded circular DNA molecules. A circular sequencing of the partially double stranded circular DNA molecules is conducted by using the probes as primers.