Low enrolment of students in the science stream after completion of high school in the state of Mizoram and prevalence of science-related anxieties within them is a matter of concern and rationalises the present study to assess the basic science process skills of high school students. It is advocated in many past studies that a good understanding of science process skills indirectly helps school students to comprehend the nature of science well, which otherwise seems very abstract. The present study tried to construct a tool that could measure high school students' understanding of basic science process skills. The tool was validated using inter-rater reliability by seven experts from higher education in the field of science. Also, the tool seems to provide a high reliability. The sample comprised 1213 high school students from five randomly selected districts of Mizoram, which are Aizawl, Siaha, Khawzawl, Serchhip and Mamit. The findings of the study reveal that students are having an above-average level of understanding of basic science process skills and do not significantly differ in their understanding with regard to gender; however, they differ significantly with regard to the school management and school locale, showing the private school students to be better than government school students and urban school students to be better than their rural counterparts. The findings indicate a need for immediate reform in high school science teaching and learning processes.