The medium effect of nucleon-nucleon cross section σmedNN(α) on the isoscaling parameter α is investigated for two couples of central nuclear center reactions　40Ca+48Ca and　60Ca+48Ca， 112Sn+112Sn and 124Sn+124Sn with isospin dependent quantum molecular dynamics at beam nucleon energies from 40 to 60MeV. It is found that there is very obvious medium effects of nucleon-nucleon cross section　σmedNN(α) on the isoscaling parameters α. The mechanism of the above properties is also studied and discussed.