This study had two main goals, and thus, underwent two phases. First, it focused on the development of The School Botanical Garden Instructional Guide, an instructional material in using the school botanical garden. It also attempted to document the steps in preparation of the developed instructional materials and evaluate its adequacy. Second, it ascertained the effect of utilizing the school botanical garden with the aid of the developed School Botanical Garden Instructional Guide. The study employed developmental and quasi-experimental designs to ascertain the students' test performance in the lessons on Plant Structures and Functions. The subjects were two purposively sampled heterogeneous sections of second year high school students belonging to the regular classes of Iloilo City National High School in SY 2008-2009, and were assigned as control and experimental groups using the flip-coin method. The experimental group was taught utilizing the school botanical garden with the aid of the developed School Botanical Garden Instructional Guide and textbooks in garden laboratory activities. The control group was taught utilizing only the assigned textbooks in typical classroom laboratory activities. The class receiving the other treatment based on their first grading performance in their biology subject before the experimentation. The researcher utilized all possible scores of the students who were able to take both the pretest and postttest. Of their bigger class sizes, the researcher was able to utilize the scores of 26 students from the experimental group, and match them with the scores of 25 students from the control group. Nevertheless, the researcher made sure that the mean first grading performance of the experimental group was similar to the mean first grading performance of the control group by establishing the equality of their mean grades before the start of the experiment. The data were gathered through the use of validated pretest and posttest. The conduct of the study lasted three weeks. The researcher requested one biology teacher to administer the tests and handle both classes at different morning schedule. Descriptive statistics used were the mean and standard deviations. To test the significance of the difference among the variables, the researcher employed the non-parametric Mann-Whitney and Wilcoxon signed Ranks tests set at .05 alpha level. The findings showed that the developed School Botanical Garden Instructional Guide was rated by the jurors to be "adequate" in terms of all its five criteria for evaluation. The two groups obtained average pretest performance. A not significance performance was noted between their performance in the pretest indicating that they were comparable before the experiment. After the conduct of the experiment, both groups reflected average posttest performance. A significant difference still existed between their posttest performance even though it was the same as their performance in the pretest (average). A significance difference was noted in each group's pretest-posttest performance. A significant difference was as well noted in the mean performance gains between the two groups.