Abstract: 【Objective】This study aimed to reveal the characteristics of morphology and structure of blackberry fruit related to firmness during fruit development and ripening.【Methods】Soft-fruited(‘Boysen’)and firm-fruited(‘Arapaho’)blackberry cultivars which had similar growth periods but significant hardness differences as research materials were used to comparatively investigate the mesocarp cell structure characteristics of fruits at different developmental stages by paraffin section.【Result】At the initial phase of fruit coloring, the exocarp and mesocarp cells of the both cultivars were arranged neatly and tightly and the cell structures were clear and intact. During the stage of fruit coloring into majority red, pericarp cells of ‘Arapaho’ fruits quickly elongated and became large as ovoid shapes, cell wall became thinner, but the cell morphology remained intact. During the process of fruit coloring into complete red, pulp cells of the both cultivars expanded continuously, cell walls of partial pulp cells dissociated obviously and the cell boundary exhibited ambiguity after 33 days after flowering. At the time of fruit ripening with full black, only exocarp cells and one layer of adjacent mesocarp cells were intact in ‘Boysen’ fruits, and the other cells inside were almost completely lysed, the cell wall lysis extent in ‘Arapaho’ fruit was really lower than that in ‘Boysen’.【Conclusion】The changing dynamics of the pulp cells during fruit development in two blackberry cultivars with differential firmness were similar before ripening. At ripening stages, cell wall lysis degree of mesocarp cells of them demonstrated an obvious difference, which was coincided with the fruit firmness, implying a vital role of cell wall lysis degree of mesocarp cells in fruit firmness determination.