We report on isotherm ( T = 77.5 K) DC magnetization hysteresis loops of a series of YBa 2 -x Sr x Cu 3 O 7 - ¿ (x = 0, 0.02, 0.1, 0.25 and 0.37) single crystals with the purpose of studying the influence of chemically introduced site disorder on the YBa 2 Cu 3 O 7 - ¿ critical current density, J c ( B , T ) and normalized pinning force density, f(h ). The Sr ion chemical disorder is inserted at YBa 2 Cu 3 O 7 - ¿ structure by lattice distortion. Transmission electronic microscopy (TEM) of the doped samples shows a structure constituted by a high density of twins, probably, decorated by many small precipitates. The J c ( B , T ) transported by the doped samples with x ¿ 0.10 is higher than that transported by the YBa 2 Cu 3 O 7 - ¿ pure sample . In contrast, the J c ( B , T ) transported by the doped samples with x > 0.10 is significant and systematically lower than that transported by the un doped sample. The analysis of the f(h ) plots indicates, in the light of Dew Hughes model, that the preponderant pinning mechanism of YBa 2 -x Sr x Cu 3 O 7 - ¿ single crystals with x ¿ 0.10 is the core normal point type. However to the samples with x > 0.10 , the f(h ) analyses indicates that the preponderant pinning mechanism undergoes a crossover from the core normal point type to the core normal surface type. This crossover of pinning mechanism dynamics could be appointed as the possible reason of the decrease of the J c ( B , T ) observed in the samples with x > 0.10

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