This paper aims to investigate the effect of gender (male & female) and nationality (India, Pakistan & Zimbabwe) on achievement of secondary school students in science and mathematics based on annual final assessment taken by the respective education boards of the participated countries. The achievement data (n=6000) for session 2012-2013 was gathered manually in 2013 from the participated schools of all the countries where each country contributed 2000 sample of students gathered from 20 schools. The statistical tests such as t-test and analysis of variance was applied for analyzing the data and results of the study indicated that no significant differences exist on the achievement of students in science or mathematics in terms of gender in India, Pakistan and Zimbabwe indicates dilution of gender disparity in these three countries. Besides, nationality of the students has influenced their achievements both in science and mathematics because both male and female students of India achieved more in science and mathematics than students of Pakistan or Zimbabwe. In addition, students of Pakistan achieved more than students of Zimbabwe as they were the least achievers. Therefore, nationality had a significant effect on the achievement of students. The reason most likely is the economy of country which play a significant role in promoting schools, teachers and students’ achievement. Although, economic status or type (public or private) of the school was not examined which can provide new insights into the influence of nationality on students’ achievement.