A bottom bracket assembly (1) of a bicycle is disclosed. The bottom bracket assembly includes a shaft (2) extending along a predetermined longitudinal direction and a pair of bearings (31, 41) adapted to rotatably support the shaft (2) with respect to a housing box (10) provided in a bicycle frame, the bearings (31, 41) being mounted on the shaft (2) at opposite end portions (3, 4) of the shaft (2). The opposite end portions (3, 4) of the shaft (2) have an outer diameter (D2) between 28 mm and 37 mm, bounds being included, and the distance (D1) between the median planes (Y1, Y2) of the bearings (31, 41) along the longitudinal direction is between 80 mm and 100 mm, bounds being included.