High pressure-high temperature (HP-HT) treatment of melt-textured YBa2Cu3O7-delta at 2 GPa, and in the 800-950 degrees C pressure-temperature range for 15-30 min in contact with monoclinic pre-annealed zirconia induces: (1) the increase of the material density from 5.7 to 6.3 g/cm(3) (by 9%), (2) the increase of critical current density in the direction of c-axis of YBa2Cu3O7-delta grains from 3-3.5 x 10(3) up to 7 x 10(3) A/cm(2) (in the self-field at 77 K) while in the ab-plane it remains unchanged (10(4) A/cm(2)), (3) the increase of Vickers microhardness from 3.95 up to 5.3 GPa (estimated under the 4.91-N load). The increase in dislocation density in the (001) planes of HP-HT treated YBa2Cu3O7-delta grains from 10(8) up to 10(12) cm(-2) may be one of the reasons of the increase in critical current density. The spaces between twin domains in YBa2Cu3O7-delta before treatment were 100-150 nm. Completely detwinned wide areas or regions, where each second twin domain was narrowed to approximately 20 nm or tapered down to zero thickness within the first domain and the disappearance of 1/6 [301] stacking faults have been observed in the treated samples.