Assessment of problem-solving skills for preservice teachers is important in science instruction. The purpose of this study is to discover; (a) How are the characteristics of the Problem-Solving Essay Test Instrument (PSETI); (b) How is the validity based on experts assessment; (c) how are the validity and reliability based on trials. The research method is research and development. The study covers three stages, those are (1) planning the test, (2) trying out the test, (3) determining the validity, and the reliability. The sample of this study was 80 preservice elementary school teachers of basic science concepts. The PSETI is in the form of an essay test, consisting of 19 items. The essay test contains ethnoscience problems. Content validity based on the Aiken index is 0.84 with a good validity level. The results of the development of the PSETI instrument show 19 items met the validity and reliability of the 21 items/ compiled. The PSETI is an essay test with the scoring which is using a partial credit model based on the three categories in the polytomous data. Instrument reliability was 0.77 and it was concluded that the instruments were good to measure problem-solving skills of preservice teachers in science learning context ethnoscience.