An extrapolation chamber was designed and constructed, and its performance was studied in beta radiation fields. The characteristics of this chamber and the measurement procedures used for the calibration of beta-ray applicators are discussed in this work. The surface dose rate of a planar Sr-90 + Y-90 applicator was also obtained using the extrapolation chamber, and the results were compared with the Amersham calibration certificate, An agreement within 2.7% was verified.