Understanding sex determination in the Pacific oyster Crassostrea gigas, a sequential hermaphrodite, can provide prospective on the evolution of sex-determining systems for comparative reproduction from an evolutionary perspective. Surprisingly, this mechanism is still poorly understood. To date, sex ratio and sex change have never been studied at the individual level for a large size group and long-term monitoring. To this purpose, we performed an ambitious individual long-term follow-up (6 years) on a large population (cohort 1: 7488 oysters) produced from wild oysters, as well as for a second population produced from the cohort 1 (cohort 2: 4320 oysters). All oysters were individually sexed from 2014 to 2019. For the cohort 1, our results showed a significantly female-biased sex ratio each year, ranging from 61 to 73% for the cohort 1. The proportion of oysters exhibiting sex change between the first two breeding seasons was 34% and decreased each year, ending at 9% between years 5 and 6. From the initial population, 1386 oysters were sexed six years in a row. Among them, 58% were sequential hermaphrodites, within which 32% changed sex once (19% protandric and 13% protogynic), 19% twice, 5% three times, 1% four times and 0.1% five times. In contrast, 42% never exhibited a sex change, within which 34% were potentially true females and 8% potentially true males. However, a logistic regression model indicates that those oysters could experience one sex reversal in subsequent years resulting that all oysters of our population of C. gigas would be sequential hermaphrodites. Similar results were observed for the cohort 2, although the proportion of sequential hermaphrodite was higher than the one observed for cohort 1. It is supposed that a genetic basis exist for sex change in C. gigas. Our work participates to unravel the barriers existing about the sequential hermaphroditism, the protandry and the sexual system in C. gigas, still currently debated.