The aimed of this research was to know the influence of the guided inquiry instruction on students learning outcomes in sub material of environmental contamination in class VII of SMP Negeri 3 Meliau. This research was a Quasi Eksperimental Design with nonequivalent control group design. The research samples is class VIIC as the experimental class and class VIIB as the control class. The instrument used was the multiple choice test with a total of 20 items. The average score of students laerning outcomes the experimentasl class taught using guided inquiry instruction was 17,33 which was higher than the control class taught using conventional instruction at 15,37. The results of analysis t test with 0,05 signification shows that guided inquiry instruction giving the influence to students learning outcomes. The value of effect size was i.e. 0,7 which was in the medium category and contributed to 27,34%
