To recognize the creative thinking of students in problem solving mathematics needed the task which can really identify it. The task has to be suitable with the objective or the goal to be measured, and fulfill the validity and reliability as an assessment instrument. Descriptive-Qualitative research is conducted to understand that the designed task is valid and reliable. The rational analysis is done descriptively from the written data which is made by evaluator (researchers, colleagues and teachers) and from interview to two students at 11th grade of SMK Senopati Sedati. Its result is the “standard†prototype of task as an assessment instrument to identify the mathematical creative thinking of student and also some others findings.