Electron contamination due to the interaction between radiation beam and material was analyzed for the factors such as source-skin distance (SSD), field size, tray characteristics and position of filter, which can affect the surface dose in Cobalt teletherapy. Surface dose in open beam was more influenced by SSD with increasing field size. Relative surface charge (RSC) increased with the use of tray (solid, circular hole, slotted), compared with open beam, which is thought to be due to increased electron contamination of the tray. To reduce the surface dose, 0.4mm thick Lipowitz metal filter was used. Compared with open beam, RSC decreased by $8.8\%,;11.3\%,;13.3\%,;16.6\%,;19.3\%;and;21.7\%$ for the field size of $5 imes5,;10 imes10,;15 imes15,;20 imes20,;25 imes25;and;30 imes30cm^2$, respectively. On the contrary, use of Lipowitz metal filter increased RSC at 60cm or less SSD. Surface dose was effectively reduced with Lpowitz metal filter placed right below solid tray in Cobalt teletherapy