In the fine-grain TL dating the full α dose must be converted into the equivalent β dose. The conversion is finished by K_eff-value, which is an effective α effectiveness. But the K_eff can not be measured directly for each sample and only the external radiative efficiency K_3.7 can be measured. In order to obtain the K_eff a special study for the conversion factor of K_eff to K_3.7 has been made using the ultrathin TLD. The results show that the conversion factor of the TLD for archaeological samples is 0.847, which is in agrcement with calculated value 0.85.