An apparatus to measure the critical pressure, density, and temperature by visual observation of the meniscus in an optical cell is presented. The critical parameters and vapor pressures for 1,1,1,2-tetrafluoroethane (R-134a) were measured to confirm the reliability of the measurements with the present apparatus. The critical pressure, density, and temperature : of 1,1,1-trifluoroethane (R-143a) were determined to be 3.7639 MPa, 434 kg.m(-3), and 345.861 K, respectively. Thirteen vapor pressures of R-143a were obtained at temperatures from 263 K to the critical temperature. The uncertainties of pressure, density, and temperature measurements are estimated to be within +/-2.6 kPa, +/-2 kg.m(-3), and +/-6 mK, respectively.