Number and algebra are important for learners of all ages. However, mastery of number and algebra skills, especially for children with learning disabilities or low mathematics achievement, are challenging and require their great efforts. This research aimed to develop a smartphone application for learning number and algebra, to investigate the efficiency of the developed application, to determine students’ achievement and improvement in learning number and algebra as well as their satisfaction towards learning through the application. Utilizing two steps of purposive technique, 104 primary school students with learning disabilities or low mathematics achievement in three southernmost provinces of Thailand were recruited to participate in the study. The efficiency of the developed application was determined by the criteria of E1/E2 equal to 80.00/80.00. Data in pre- and post-test were analyzed with t-test to examine a significant improvement after the students learned number and algebra through the smartphone application while the students’ responses in questionnaire were analyzed with descriptive statistics (i.e., mean [M] and standard deviation [SD]) to determine their satisfaction towards learning through the application. Interview sessions were conducted with 10 teachers to gain an insight into how the students enhanced their number and algebra skills. The results showed that the developed application was efficient at 82.32/84.53 as compared with a standard criterion of 80.00/80.00. Moreover, the students’ post-test score was significantly greater than pre-test and greater than 60.00% criterion at the significant level of 0.05. The results also indicated that the application showed positive effects on the students’ improvement in number and algebra skills and their satisfaction towards learning through the application, which was at high level (M=3.97, SD=0.63).