Real-time quantitative PCR (RQ-PCR) is a recently developed technique for nuclear acid quantification, in which conventional PCR and probe hybridization are integrated together. A specific fluorescent-labeled probe, binded with DNA/cDNA target, is supplemented into the reaction system of PCR. During the extension phase of PCR amplification, the specific fluorescent-labeled probe is cleaved by the 5'-->3' exonuclease activity of Taq polymerase, producing a fluorescent signal that can be detected. The amount of fluorescence released in the reaction is proportional to the target sequence number. Thus, an exactly and absolutely quantitative assessment of initial target DNA/cDNA copy number can be made in a closed tube assay system. This technique eliminates the need for post-PCR sample processing which is necessary in conventional PCR and decreases the probability of contamination which is inevitable in conventional PCR. Moreover, it reduces the potiental for false positive or negative results by utilizing the hybridization of a probe to sequences internal to amplification primers. RQ-PCR is proved to be a sensitive, specific, reliable and convenient tool for the quantification of minimal residual diseases (MRD) of hematological malignancies by recent studies. This article reviews the principle of RQ-PCR and its application in quantitative detection of MRD of hematological malignancies such as acute myelogenous leukemia (AML), acute lymphoblastic leukemia (ALL), chronic myelogenous leukemia (CML) and malignant lymphoma.