This paper presents a method to for testing optical transmitances of the X-ray filters as low as 10

-7

～10

-8

/cm

2

. In this method we used a He-Ne laser operating at 632.8nm wavelength as a light source. In order to measure such low transmitances

we designed and equipped a ultrolow transmitance measurement device of retlection mode

using a photomultiplier as the detector

a digital voltmeter as the reading device. Moreover

we also used a group of grey absorbing filters as the transmiting attenuator to measure the ultralow transmotances. Both methods have given satisfactory results.