Endocarp lignification is important fruit growth phenophase since after its completion fruit starts with oil accumulation. The information about duration of endocarp lignification is important for timing of management practices, irrigation and pest control in oil cultivars, and fruit thinning in table cultivars to obtain uniform fruit weight and size. In this study, fruit length, width and weight of four Istrian autochthonous olive cultivars (‘Bu a’, ‘Punto a’, ‘Ro injola’ and ‘Istarska bjelica’) were measured. Samples were taken from olive collection orchard of the Institute of Agriculture and Tourism in Pore in equal growing conditions. The aim of the research was to define an olive fruit growth dynamics during pit hardening. Fruit weight in all cultivars was increasing during endocarp lignification (from 7th to 28th July). The highest percentage in the weight growth in the first week had cultivars ‘Bu a’ (48.5%) and ‘Ro injola’ (44.6%) while in the second week maximum was reached by cultivars ‘Punto a’ (44.2%) and ‘Istarska bjelica’ (42%). The highest increase in total fruit mass was detected in ‘Punto a’ (1.30 g) and the least at ‘Ro injola’ (0.56 g). Maximum increase in length (L) and width (W) had ‘Punto a’ (L: 7.13 mm; W: 4.23 mm) and the least ‘Istarska bjelica’ (L: 2.48 mm; W: 2.70 mm).