This research entitled "Mathematical Basic Ability Test in the 3rd grade of elementary school (A Case Study in Kebumen)" aimed (1) to describe the result of Mathematical Basic ability Test in the 3 rd grade of Elementary School in Kebumen Regency, (2) to describe the mastery level of Mathematical Basic Ability Test achieved by the 3 rd graders of elementary school in Kebumen Regency, (3) to describe the type of errors made by the 3 rd graders of Elementary School in Mathematical Basic Ability Test. The research approach employed was descriptive. This research was a case study conducted in Kebumen Regency area in 2018. The subject of research consisted of 871 3 rd graders in 40 elementary schools distributed in 5 sub districts in Kebumen Regency area. Data source employed was 3 rd graders and teachers of 3 rd grade of Elementary School. Meanwhile, techniques of collecting data used were test, interview, and observation. Data validation was carried out using source and technical triangulations. Data analysis was conducted using qualitative analysis technique encompassing data reduction, data display, and conclusion drawing or verification. The result of research showed that: (1) the ability of 3 rd graders of elementary school in Mathematical Basic Ability Test in Kebumen Regency reached mean score of 39.24; (2) the mastery level of Mathematical Basic Ability Test achieved by 3 rd graders of elementary school in Kebumen Regency has mean score of 40.46%; (3) the type of errors made the 3 rd graders of elementary school in Mathematical Basic Ability Test were: (a) writing, reading name of and symbol of number, (b) ordering number, (c) place value, (d) subtracting number, (e) multiplying 2 numbers, (f) dividing number, (g) mixed count operation, (h) comparing, adding, and subtracting fraction with same denominator, (i) counting currency value, (j) square, rectangle, angle, (k) measurement of length, width, weight, and time.