In vivo dosimetry is the best way to confirm accuracy of delivered to the patient. Proper calibration of TLD prior in vivo measurement required to ensure accurate dose measurement. TLD100 calibration were done using Alcyon cobalt60 teletherapy machine using SSD technique on water phantom. Series of doses from 10cGy to 400cGy delivered to determined TLD response over dose variation and calibration factor. Cobalt 60 radiation output calibration was measured to determined irradiation time. TLD's were placed on phantom surface and in water at 5cm depth using 10cm X 10cm field size, and read using Harshaw Model 2000A and 2000B TLD reader. Depth of maximum was selected as TLD dosimetry reference point, and calibration factor and curve were made based on dose at dmax position. To study TLD response with field size changes, the TLD were irradiated with several field sizes. Measurements showed different result between two calibration methods. On surface calibration over series of doses giving linier function of y = 85.732x - 1.874 and in phantom measurements giving linier function of y = 52.388x + 14.749. The calibration factor for on surface and in phantom are 84.365 cGy/μC and 57.158 cGy/μC. TLD show high response at 4cm x 4cm field size and show decreasing trend as field size increases. Calibration should be done according to invivo measurement setup that will employ at the institute. It is clearly seen that the two different setup gave greatly different results. Surface calibration suitable for dose estimation using entrance dose measurement setup. In phantom calibration only suitable for dose determination that uses TLD within phantom, such as IMRT dose verification using TLD or organ dose study, and it is important to correct the TLD response with field size changes.