Background: In gamma-ray spectrometry for the radioactivity analysis of bulk samples, self-attenuation correction is necessary when the attenuation rates in test sample differ from the one in the efficiency calibration source. Thus, the mass attenuation coefficient of test samples is an important factor in gamma-ray spectrometry. This study estimates the mass attenuation coefficients for seven kinds of environmental samples. Materials and Methods: An uncollimated transmission system with a high purity germanium detector system was used to measure the gamma-ray transmission rates with and without test samples. The system was calibrated using C (activated charcoal), H₂O, MnO₂, NaCl, Na₂CO₃, and (NH₄)₂SO₄ as reference materials. Sea sediment, surface soil, fish, seaweed, Chinese cabbage, milk, and pine needles were selected, and ten identical samples for each sample type were tested using the system. Results: The calibration of the uncollimated transmission system was validated with good agreement within 4% between linear attenuation coefficients by experiment and calculation for K₂CrO₄ and SiO₂. The standard deviation of the mass attenuation coefficients for each kind of sample was estimated as less than 5% above around 100 keV. Conclusion: Mass attenuation coefficient does not significantly depend on the sample type for gamma-ray energy higher than around 100 keV, but mass attenuation coefficient for the lower gamma-ray energy should be considered even with similar kinds of sample. The mass attenuation coefficients tabulated in this paper can be used as a reference or comparable value in gamma-ray spectrometry for environmental samples.