The objective was to study the effect of cooperative learning strategy of NumberedHeads Together (NHT) on critical thinking ability in Biology of the tenth grade-students at theState Senior High School of SMA Negeri 1 Wori in North Minahasa Regency. Method of theresearch used the quasi experiment design of pretest-posttest nonequivalent control group design.Populations of the research were the tenth grade-students from four classes, and samples of theresearch were two classes, class Xa (experiment) and Xb (control), which included 60 students.Data of the research was collected by pretest-posttest technique and using critical thinking abilitytest. And then, data ofthe research was analyzed using descriptive statistics and anacova statisticalanalysis. Result of the research showed the effect of NHT-cooperative learning strategy on theincreasing ability ofcritical thinking ofthe students in comparison with the conventional one.