This paper deals with the properties of films used as entrance window of a gas-flow type ultrasoft X-ray counter. The effects of the base material and conductive coatings of the films on performance characteristics of the counter are investigated. Experimental results show that, among the material under study, Formvar exhibits more satisfactory gas-sealing property and has higher transmissivity for X-rays. In addition, it is found that the conductive coating on the film has a significant effect on the stability of the measured intensity. An interpretation on these effects by taking into acounts of the feature of electron multiplication process in the counter caused by ultra-soft X-rays is proposed.