An investigation into the effect of fluctuations in environmental conditions on the degradation in the form accuracy of finished product is carried out when using ultra-precision aspheric generator with laser interferometric measurement system. It deals mainly with the profile error of finished surface and the relative displacement between tool and workpiece during raster flycutting. In order to correlate the surface profile with the change in air pressure, cutting experiments are carried out in a precisely air-conditioned environment of temperature change within ±0.06°C, humidity change ±1% and natural pressure fluctuation for up to 18 hrs. The relative displacement between tool and workpiece, the nominal positions of which are fixed respectively for long period of time (144 hrs), is carefully measured and analyzed by correlating with change in refraction index of air. Experimental results indicate that the deviations of surface profile from the reference line increase in proportion to those of air-pressure drift, and that the form accuracy is determined solely by the peak-to-valley of air-pressure deviations. Comparing the measurement with the analysis, it is confirmed that the linearity in the above profile-pressure relation results from the incorrect tool positioning due to the laser measurement error in proportion to change in air pressure during machining.