We have developed a piezoelectric DNA-sensor based on DNA-RNA hybridization for the detection of two orchid viruses, Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV). Specific oligonucleotide probes modified with a mercaptohexyl group at the 5′-phosphate end were directly immobilized onto 10-MHz AT-cut quartz crystal microbalance (QCM). QCMs coated with such oligonucleotide probes were exposed to test solutions containing viral RNA for hybridization. Various experimental conditions evaluated were (i) DNA probe coating concentration, (ii) sensitivity and specificity of the probes at different hybridization temperatures, and (iii) effects of incubation temperature on the hybridization time. The specific nucleotide probe-coated QCM-based DNA sensors were able to detect both CymMV and ORSV in quantities as low as approximately 1 ng in purified RNA preparations and 10 ng in the crude sap of infected orchids. This is the first application of a DNA biosensor for the detection of plant viruses.