The research aimed to analyze the students' abilities in solving realistic mathematics problems using "What-If"-Ethnomathematics Instruments with content focused on plane and space materials. The "What-If"-Ethnomathematics instruments are instruments that enable educators to analyze various errors and obstacles experienced by students in solving realistic problems by prioritizing applied and culture mathematics and questions that test students' mathematical thinking skills. The research design was descriptive qualitative. The subjects of the research were 46 students of SMP Widiatmika in the 2020/2021 academic year. The data collection method using a test, interview, and documentation. The data was then analyzed using qualitative descriptive data analysis with the following stages: data reduction, data presentation, and drawing conclusion and verification. The result showed that the students' abilities in solving realistic ethnomathematics problems using "What-If"- Ethnomathematics Instruments" are still lacking which include: errors in understanding the problems, errors in representation, errors in reasoning, errors in answering "What-If" Questions. The highest errors were errors in reasoning, which was 69.56% of all students, followed by errors in answering "What-If" Questions of 65.21%, then errors in understanding the problems 43.47%, and finally errors representation as much as 34.78%. From the results of an interview, teachers tend to provide learning that focuses on the delivery and use of formulas and ignores the understanding of concepts and improved thinking skills of students based on realistic mathematics problem-solving.