The magnetoresistance, the Shubnikov-de Haas (SdH) effect and their orientation dependence of an organic conductor (BEDT-TTF) 2KHg(SCN) 4 have been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The SdH frequency F=670T, the cyclotron mass m c =1.4 m 0 and the Dingle temperature T D =1.6 K are obtained when the fields are applied perpendicular to the conducting a-c plane. The dependence of F on the angle θ between the field and the b ∗-axis (⊥ a-c plane), which deviates slightly from cosθ, indicates the presence of the weakly warped cylindrical Fermi surface with an interlayer transfer integral estimated to be about 1 meV. Below 1 K and above 10 T, the spin-splitting effect is clearly observed and the direct analysis of the splitting oscillations results in the series of the g-factor; g=1.4 n±0.6 ( n is an odd number). The rigorous g-factor of 2.0 ( n=1, + sign), being consistent with ESR measurements, is obtained.