Nowadays, the problems faced related with the assessment quality is the use of doubtable instrument of speaking ability assessment. Therefore, the current study is carried out to find out the appropriateness of the instrument used in assessing the group B students' speaking ability through the validity and reliability. The study is development research which is used RDR model. The data collection technique is by using validity questionnaire. Two experts are used to assess the appropriateness of the instrument. The research subject is the instrument for assessing the speaking ability of group B children which consisted of grid and questionnaire in form of 12 items. The data validity is analyzed by using Gregory and the data reliability is tested using Kuder Richardson-20 formula. The result shows that the valid questions are 10 items. Those are tested using the instrument for assessing children of group B. The validity score is 0.83 which categorized as very high criteria. Furthermore, the reliability of the instrument is 0.81 which means that the reliability is placed as very high criteria. Thus, it can be concluded that the developing instrument on the current research has high validity and reliability which means that the instrument is appropriate to use in assessing the children of group B speaking ability.