Objective:To investigate the scatter factor of Irdium192 HDR source during calibration in air.Methods:All measurements are performed with thimble 0.6cc Farmer type thimble chamber and PTW well-type chamber.The well type chamber has been placed 1 metre above the floor to guarantee negligible influence of the floor scattering.The distance of outer lateral surface of the chamber to room wall was varied in the range 5cm to 200cm.The sensitivity of the well-type chamber for scattering from room wall on different distance was investigated and then the reading at each distance has been normalized to the response for 200cm distance.The Ir192 source and thimble chamber with build-up cap were positioned on the mobile stand and the distance between the source and chamber(SCD)was varied accurately from 5cm~40cm during the calibration.In the calibration procedure,the air Kerma has been measured according the multiple distance method(Goetsch)、DGMP and shadow field method(SF)at several different distances from the center of the active core of the source and the reference point of the ionization chamber in a specially designed jig.During shadow field method(SF)the seven distances measurement is taken in two phases,the first phases is the measurement without shield,the second one with a 6cm(three-tenth value layer)cone of lead block placed as close as possible the source and fully shield the chamber.Data was analyzed and fitted with Origin 8.0 software.All correction such as leakage、end-effect and chamber gradient are required prior to measurement.Results:For calibration with well type chamber the scatter is less influence from 1 meter distance from the lateral wall of the room.The result also shows that with Goetsch and DGMP method the scatter factor is independent of SCD and negligible.With the Shadow Field method the scatter factor is less but constant basically.conclusion:The calibration have to carry out in minimum scatter environment with well type chamber at least 1 meter away from any wall or floor.For calibration in air with thimble chamber it is best to measurement the scatter in local and the SF likely will serve the better method of obtaining a scatter factor.