The aim of this study was to investigate number conservation ability among girls of co-educational (Coed) and single-sex primary schools. Two major objectives of the study were: 1) To investigate number conservation ability of girls belongs to Coed and single-sex primary schools; 2) To compare girls of Coed and single-sex primary schools on number conservation ability. The study was based on three null hypotheses: 1) Girls (age group of 3 to 5) of Coed and single-sex primary schools are not number conservers; 2) Girls (age group of 6 to 8) of Coed and single-sex primary schools are not number conservers; 3) There is no significance of difference in the frequency of Coed and single-sex primary schools? girls. All the government primary schools (Coed & single-sex) of Khyber Pakhtunkhwa were the population of the study. A total of 240 girls students from Coed & single-sex primary schools were taken as a sample of the study. Empirical research design was taken as research design. Observation sheets were developed for data collection. Statistical tools such as percentage and t-test were applied for the analysis of data. Girls (age group 3 to 5) were not found to be number conservers. It was found from the results that girls (age group 6 to 8) were number conservers. It was also found that girls from Coed schools performed better (found to be number conservers at the age of 6) than those of single-sex schools (found to be number conservers at the age of 7). Hence it is recommended that girls are to be allowed and encouraged to study in Coed (boys? primary schools), government and other agencies should take serious steps in this regard.