The timescale from 10 days to 2 months (sub–seasonal to seasonal) is important in water resource management, reservoir regulation, both irrigation and hydroelectricity, as well as agricultural scheduling in the North Central region. However, it is also impossible to deny the fact that, in Vietnam in general and the North Central region in particular, the sub season forecasting skills are still low. The authors corrected the sub–seasonal to seasonal rainfall forecast of the ECMWF model based on the quantile mapping method with gamma approximation (QM–G) for the total rainfall of 5 days with each forecast period (3–6 weeks). Two main data sets include: the ECMWF model's sub–seasonal rainfall forecast data set for 20 years in the period 2000–2019 and the TRMM satellite rain data set. The results show that: After correcting, the ME, MAE, RMSE values decreased significantly, especially from December to March next year. Besides, in July–September, although the bias has decreased compared to before the correction, there are still many areas (mainly in the southern region) that still have large bias. However, the correction by QM–G did not improve the correlation, even some month it decreased compared to before correction.