It is known that fixed source to skin distance (SSD) cannot be used when the treatment field is sloped or larger than the size of second collimator in electron beam irradiation and inverse square law using effective ssd should be adopted. Effective SSDs were measured in different field sizes in each 6, 9, 12, 15 and 18 MeV electron energy by suing NELAC 1018D linear accelerator of Kosin Medical Center. We found important parmeters of effective SSD. 1. Minimum effective SSD was 58.8 cm in small field size of 6x6 cm and maximum effective SSD was 94.9 cm in large field size of 25x25 cm, with 6 MeV energy. It's difference was 36.1 cm. The dose rate at measuring point was quite different even with a small difference of SSD in small field (6x6 cm) and low energy (6 MeV). 2. Effective SSD increased with field size in same electron energy. 3. Effective SSDs gradually increased with the electron energies and reached maximum at 12 or 15 MeV electron energy and decreased again at 18 MeV electron energy in each identical field size. And so the effective SSD should be measured in each energy and field size for practical radiotherapy.