This research was put forth to reveal the analysis of the definitions of secondary school students by qualitative phenomenological research methods about "global warming". About 166 students at a secondary school in Istanbul participated in the research within 2009 - 2010 academic year, second term. The research datum were obtained by students' completing the sentence "according to me, global warming means ...". According to the findings of the phenomenological research, secondary school students formed 6 different global warming definition categories. These definition categories are as follow: (1) Global warming is the rise of average temperature of earth, (2) Global warming is the change of climates and seasons, (3) Global warming is the destruction of natural (ecological) balance, (4) Global warming is the result of the destruction by people, (5) Global warming is an environmental problem that can be prevented if necessary precautions are taken. (6) Global warming is the end of humankind and the world. It is thought that global warming theme should be emphasized in the secondary schools curriculum and the students should be given an effective environment education.