Nowadays there are a variety of high-precision refractometers to measure the refractive indices of liquid and gaseous media. A variety of physical processes are used in modern refractometers. It is known the method of refractive index determining of the liquid by the diffraction grating (diffraction angle depends on the refractive index of the medium in which the diffraction grating is situated). However, the more precise determination of the refractive index of liquid by the diffraction grating is apparently possible not only by measuring the diffraction angle but also by diffraction efficiency measuring [1]. Diffraction efficiency η of the relief grating is defined as the ratio of light intensity of first diffraction order to the intensity of the total light incident on grating [2]. The purpose of this study is numerically-based computer calculation of the dependence of the reliefphase gratings diffraction efficiency on environment (liquid) refractive index in which the relief grating is immersed. As an investigated material we used chalcogenide material As2S3. Thin chalcogenide films were prepared by thermal evaporation in a vacuum chamber at pressure of 10-5 Torr. Thin As2S3 film (about 1.0 m) was deposited on optical glass substrate at room temperature at a deposition rate of 100 Å/s. The monitoring of As2S3 film thickness was carried out during the evaporation by interference technique. An interferometric holographic recording was used to record linear grating on the As2S3 film. CW DPSS single mode laser operated at 532 nm and power density 100 mW/cm2 was used for recording. The holographic gratings with a period of d = 1 μm were recorded by two spatially symmetrical laser beams. The intensity ratio of recording beams 1:1 was used. After recording the selective wet etching was used to obtain a relief-phase diffraction gratings. It was studied the influence of grating material on the diffraction efficiency dependence on the refractive index of the environment. It was shown that both for transmission and reflection mode there was a clear relationship between the diffraction efficiency and refractive index of the environment. We have designed an original programme that estimates and calculates the refractive index of liquid environment. The precision of the method was evaluated and proved taking the liquids with known refractive index. Studied dependence is proposed to use the in compact portable refractometers for determining the refractive index of liquids.