Experimental results indicate the previous theoretical model cannot predict well the surface roughness in single-point diamond turning on a precision lathe. In solving that, an improved model was presented in this paper. The difference between the previous model and the improved model is that the relative tool-workpiece vibration is measured before cutting operation using a capacitive displacement sensor in the previous model whilst the vibration is extracted from the measured surface profile in the improved model. The relative vibration was first studied under various cutting conditions to establish the vibration modes under corresponding cutting conditions. Then the surface roughness was predicted based on the vibration modes. The results prove that there is good agreement between the predicted values and measured values and the improved model is useful and reliable.