Two duplex polymerase chain reactions were developed for screening genetically modified plants. The certified reference materials as dried powders containing 0-5% Roundup Ready soybean and maize MON-810 were used for optimization and validation of PCR methods. The genomic DNAs were extracted by Qiagen DNeasy plant mini kit and analysed using species-specific and GMO-specific uniplex PCR. The PCR products were evaluated by agarose gel electrophoresis. Different combinations of the GMO-specific primers were verified for their suitability for duplex PCR. Two duplex PCRs were developed and optimized, that allow simultaneous detection of two common transgenic elements, such as: cauliflower mosaic virus (CaMV) 35S promoter and Agrobacterium tumefaciens nopaline synthase (NOS) terminator. The developed duplex PCRs are effective tools for rapid and cheap screening of transgenic plants.