Purpose To evaluate the reliability and failure modes of ultrathin (0.5 mm) lithium disilicate, translucent and ultra‐translucent zirconia crowns for posterior teeth restorations. Materials and Methods Fifty‐four mandibular first molar crowns of three ceramic materials: (1) Lithium disilicate (e.max CAD, Ivoclar Vivadent), (2) 3Y‐TZP (Zirconn Translucent, Vipi), and (3) 5Y‐PSZ (Cercon XT, Dentsply Sirona), with 0.5 mm of thickness were milled and cemented onto composite resin abutments. Eighteen samples of each group were tested under mouth‐motion step‐stress accelerated life testing in a humid environment using mild, moderate, and aggressive profiles. Data was subjected to Weibull statistics. Use level curves were plotted and reliability was calculated for a given mission of 100,000 cycles at 100, 200, and 300 N. Fractographic analyses of representative samples were performed in scanning electron microscope. Results Beta (β) values suggest that failures were dictated by material's strength for lithium disilicate and by fatigue damage accumulation for both zirconias. No significant differences were detected in Weibull modulus and characteristic strength among groups. At a given mission of 100,000 cycles at 100 N, lithium disilicate presented higher reliability (98% CB: 95–99) regarding 3Y‐TZP and 5Y‐PSZ groups (84% CB: 65%–93% and 79% CB: 37&–94%, respectively). At 200 N, lithium disilicate reliability (82% CB: 66%–91%) was higher than 5Y‐PSZ (20% CB: 4%–44%) and not significantly different from 3Y‐TZP (54% CB: 32%–72%). Furthermore, at 300 N no significant differences in reliability were detected among groups, with a notable reduction in the reliability of all materials. Fractographic analyses showed that crack initiated at the interface between the composite core and the ceramic crowns due to tensile stress generated at the intaglio surface. Conclusions Ultrathin lithium disilicate crowns demonstrated higher reliability relative to zirconia crowns at functional loads. Lithium disilicate and zirconia crown's reliability decreased significantly for missions at higher loads and similar failure modes were observed regardless of crown material. The indication of 0.5 mm thickness crowns in high‐load bearing regions must be carefully evaluated. Clinical Significance Ultraconservative lithium disilicate and zirconia crowns of 0.5 mm thickness may be indicated in anterior restorations and pre‐molars. Their clinical indication in high‐load requirement regions must be carefully evaluated.