Research on mathematics teacher professional development has emphasized primarily teaching quality enhancement. Mathematics task as one of the vital components in mathematics teacher education has been sparsely investigated. The purpose of this research is to explore mathematics teachers' beliefs and practices in applying a problem-based task after attending one-semester online teacher professional training funded by the Ministry of Education. This is a mixed-method research design with data collected by distributing questionnaires to 105 mathematics teachers from 24 provinces in Indonesia after accomplishing an online professional training program to determine their self-reported beliefs about mathematical problem-based tasks. Data were also collected by conducting focus group interviews with 15 mathematics teachers with different teaching experiences. Teachers' relevant documents, such as course contents, task forms, and notes, were analyzed and used to enrich the qualitative interpretation. The quantitative result indicated that mathematics teachers' beliefs on the problem-based task were low. Meanwhile, the qualitative findings indicated this category of teachers constructed their problem-based tasks with no theoretical framework. Three misconceptions in designing problem-based tasks were also reported. Therefore, future research is recommended for a more acceptable and applicable problem-based tasks framework and design a specific teacher professional development training to promote teachers' competencies.