The determination of preformation factor was successively done for 343 nuclei: 212Po, 105 even-even, 85 odd-even, 78 even-odd and 75 odd-odd nuclei using the representation of clusterization states (RCS) and cluster-formation model (CFM) (Saleh Ahmed, 2017). As CFM well described the alpha clustering of the ground states of the nuclei, it was considered as bases of describing the effect of the last unpaired nucleon. The typical nucleus 212Po and the nuclei 213Po, 213At and 214At, or Po isoalpha group, which have the same alpha cluster were chosen to evaluate the hindrance of alpha clustering by their last unpaired nucleons. The hindrance was expressed in an initiated factor, named “alpha clustering hindrance factor”. The results agreed with others that have preformation factor using different methods and used to reproduce more accurate alpha decay half lives using Viola-Seaborg semiempirical formula for odd-even Po isotopes The alpha-formation hindrance factor showed an interesting behaviour that plays an important role in the provision of more information about the nuclear structure.