This study attempted to determine the minimum number of cells required to conduct DNA analyses effectively. Oral mucosal cells obtained from eight persons were suspended and individually collected by using micromanipulation technique. DNA was extracted and amplified by whole-genome amplification (WGA). Nuclear DNA was extracted to evaluate the feasibility of autosomal short tandem repeat (STR) polymorphism and Y-chromosomal STR polymorphism analyses. Tests were conducted with 20 and 30 cells, to determine the minimum number of cells required for each DNA analysis. Tests with 20 cells were repeated 5 times, to examine reproducibility. When five or 10 cells were used, loci could not be identified for most alleles. Furthermore, DNA polymorphism analyses of a single cell transferred directly to a polymerase chain reaction solution were unsuccessful. The present findings suggest that, in forensic identification, 20 or more cells are required in order to obtain clear results from autosomal and Y-chromosomal STR polymorphism analyses. Furthermore, the feasibility of sample preservation and reexamination was also confirmed by DNA amplification with WGA.