In a method of deducing the number of repeat units in a selected short tandem repeat (STR) in a genomic sample, at least a single stranded target DNA generated from a genomic sample comprising a selected STR, an STR probe (P1,P1′), a reference probe (P2), and two blockers (B1,B2) are provided, and at least three differential hybridization experiments are carried out, based on which the number of STR probe oligonucleotides (P1,P1′) bound per target DNA strand in each differential hybridization experiment is determined. The method further comprises the step of comparing these numbers of STR probe oligonucleotides (P1,P1′) bound per target DNA strand in the differential hybridization experiments for deducing the number of repeat units in the selected STR on the single stranded target DNA strand. Also disclosed are kits for carrying out STR genotyping by differential hybridization.