Abstract: The present invention relates to a method for constructing internal standards used in competitive polymerase chain reaction (PCR) for determining the amount of a target nucleic acid sequence in a sample. The method comprises the steps of a) digesting the cloned target nucleic acid sequence with a cohesive-end generating restriction endonuclease; b) filling the restriction endonuclease cohesive-ends of step a) by Klenow fragment of DNA polymerase; and c) blunt-end ligating the fragment of nucleic acid of step b) to form the internal standard. The internal standard nucleic acid sequence differs from the target nucleic acid sequence such that a restriction endonuclease recognition site is destroyed by adding sufficient contiguous bases to the target nucleic acid sequence such that the size difference between the target nucleic acid sequence and the internal standard nucleic acid sequence can be detected by gel electrophoresis.