Three ultra-precision machining processes namely fast tool servo, slow tool servo and diamond milling, are frequently used to produce optical freeform surface. Slow tool servo machining has the advantages of no extra attachment and fast setting-up, however the three dimensional tool shape compensation and tool-path generation must be conducted carefully for getting high form accuracy and fine surface finish. This research aimed to develop a model of three dimensional tool shape compensation for generating 3D tool path in slow tool servo diamond turning of asymmetrically toric surface for astigmatic contact lens. The form accuracy of freeform surface was measured by ultra-high accuracy 3D profilometer (UA3P) with user define function. After correction, the form error is less than 0.5μm both in X- and Y-direction and the surface roughness is less than 5nm.