Abstract: The halfvalue layer(HVL) of the broadbeam Xray is often used to calibrate the energy of an electron linac, but in some cases the measurement of the broadbeam Xray HVL is not convenient. Therefore, the measurement of the narrowbeam Xray HVL as a method to calibrate the energy of an electron linac is discussed in the paper. The MonteCarlo code, MCNP4B is used to calculate and analyze the narrowbeam Xray HVL, meanwhile the narrowbeam Xray HVLs are measured for the 2.5,4,6 and 9 MeV  electron linacs. There is a good agreement between the measured HVLs and the MCNP4Bcalculated results. Also, the length of a primary collimator, the width of a collimator slot, the thickness of a target and such other factors' influence on HVL is discussed, and some useful suggestions are given to reduce the measuring errors of the narrowbeam Xray HVL used to calibrate the energy of an electron linac.