Thickness shear mode Bulk Acoustic Wave (BAW) resonators with frequency of 6 MHz, were fabricated using monoclinic piezoelectric crystals PrCa4O(BO3)3 (PrCOB) and NdCa4O(BO3)3 (NdCOB). Zero temperature coefficient of frequency (TCF) characteristics were achieved over the temperature range of -140 °C to 200 °C for (YXt)-1.5° cut PrCOB and (YXt)15° cut NdCOB, with the turnover temperature at 20 °C. The electromechanical coupling factor k26 and the piezoelectric coefficient d26 were determined to be 30.2% and 15.8 pC/N for PrCOB, 29.0% and 15.1 pC/N for NdCOB resonators, respectively. The temperature independent frequency behavior, large coupling factor, high piezoelectric coefficient, together with noticeable mechanical quality factors (Q > 2,500), make PrCOB and NdCOB crystals good candidates for sensing applications with expanded temperature usage range. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.