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tokens: of horizontal field changes in shear region P is longer and followed by a flatter gradual increase. 4. Summary and Discussions We have presented a detailed study of the structural evolution of photospheric flow and magnetic fields associated with the 2015 June 22 M6.5 flare, concentrating on the central penumbrae of the δ region around the flaring PIL. We tracked the penumbral flow field with the DAVE method using high-resolution TiO images from BBSO/GST, and also analyzed the associated magnetic field using SDO /HMI vector magnetograms. The main results are summarized as follows. 1. Beginning from the first HXR peak, one segment of penumbra lying in the negative field experiences a distinct grow, extending northward at 1.5(±0.05) km s −1 for a distance of 26(±3); meanwhile, the flow velocity within the entire penumbral region becomes more pronounced, gaining a 115(±10)% increase reaching up to 2.2(±0.11) km s −1 at ∼90 minutes after first flare peak. These structure and flow field evolutions are accompanied by a step-like increase of horizontal magnetic field by 150(±15) G and decrease of inclination angle by 5(±0.5)° in 18 minutes from 17:50 UT to 18:08 UT. 2. A region of shear flow at the flaring PIL is found next to the location of a flare precursor brightening, and expands significantly after the flare. From the first HXR peak, the shear flow velocity increases gradually by 0.4(±0.1) km s −1 from 0.5(±0.05) km s −1 at 17:30 UT to 0.9(±0.05) km s −1 at 19:30 UT. As for magnetic field properties in the shear flow region, the horizontal field strength increases 220(±20) G while the inclination angle decreases 5(±0.5)° from 17:50 UT to 18:20 UT, both following a stepwise fashion. In comparison with the extending penumbral segment, the transition time of step-like magnetic field changes in the shear flow region is ∼10 minutes longer. Our results of the strengthening of penumbral structure at the center of the δ region, the associated increase (decrease) of horizontal field (inclination angle), and their close timing relationship to the flare energy release strongly favor the back-reaction of coronal fields as the underlying cause, which results in a more horizontal configuration of the photospheric field. While most other work only deals with the flare-related overall intensity change of penumbral structure, we are able to study the evolution of penumbral flow field owing to the high-resolution BBSO/GST data. We show that, although noticeable intensity darkening is only observed in the newly formed penumbral portion, the strengthening of penumbra is predominantly manifested as the enhancement of penumbral flow velocity and enlargement of penumbral area. The inconsistency of our results with the claim of darkening of the central penumbral feature in some previous studies may be due to the limitation of the low-resolution images used (e.g., Liu et al. 2005 ) and also the complexity of the central penumbral area near the PIL (Wang et al. 2017