The invention relates to an oligonucleotide which serves as a nucleic acid probe comprising a loop section with a loop sequence complementary to the target sequence of the nucleic acid molecule; a stem section arranged on both ends of the loop section for hybridizing with each other for closing the oligonucleotides thereby forming a loop, and wherein one of the stem sections is labeled with fluorescent color, such that when the loop section hybridizes with the target sequence of the nucleic acid, the oligonucleotide opens and the distance between the fluorophore and a quencher nuleoside increases which thereby prevents quenching of the fluorescence and the strength of fluorescence when measured indicates the presence of the nucleic acid with the target sequence.