Release of the PISA 2018 results revealed that Macao is not only a high-performing but also fast- improving economy in three core literacies (reading, mathematics and science) in past decade. Based on the publicly released data this study seeks to validate that Macao’s remarkable progress in one main part is due to the lowering of prevalence of grade repetition in past decade. Secondary data analysis of the Macao-PISA data was conducted, analyzing the trend of the rates of student grade repetition against the backdrop of the percentage of Macao’s 15-year-olds studying at the various grade levels. The trend over the six cycles of Macao-PISA assessment since 2003 is that there is an increasing percentage of 15-year-olds studying at grade levels appropriate to biological age with same-age peers. Admittedly, when more of the sampled students are studying at the modal grades more of them possess favorable psychological dispositions and self- regulatory learning mechanisms. Consequently they are able to reach or even excel beyond baseline performance standards. Thus, grade level effect of an age-based sample provides one convincing explanation on why Macao is commended being a fast-improving and high-performing economy in PISA 2018 in the past decade.