Dependence of the retention of crowns at their axial wall on the film thickness of zinc phosphate cement and the taper angle was investigated. Stainless steel dies, composed of a preparation and an occlusally perforated crown the taper angle of which was 2.9, 5.7, 8.5 and 11.3 degrees respectively (Fig. 1, (a) and (b)), were cemented within three min. after the start of mix with a Shofu Micro Cement having a powder-liquid ratio of 1.5 g/0.5 cc. The thickness of the cement layer at the axial wall was regulated to a given value between 9 and 55 mu through the use of a discrepancy measurer. A Shimazu Autograph tensile test machine was employed to measure 48-hr retention (kg/cm2). The retention of the crowns was strongly dependent on the cement film thickness and the taper angle when the film thickness was below a critical value (15 mu in this experiment), but not when the film thickness was above this value (Fig. 3). This may be explained by the term of mechanical interlocking of either of the unreacted powder grains and the matrix phase in the luting cement, as schematically drawn in Fig. 6. The same findings were more evidently appeared on the semilogarythmic diagram in Fig. 4. In this diagram, the critical value was shown as intersection of two straight lines which could be drawn through all the measurements for four taper angles, using the least squares.