* As a subject: It should be written Natural Science, capitalized like other subjects (e.g., English, History, Math).

* As a general term: It should be written natural science, lowercase like other general terms (e.g., social science, physical science).

Here are some examples:

* Natural Science is a broad field of study.

* She majored in Natural Science at university.

* The study of the universe falls under natural science.

* The museum has a special exhibit on the history of natural science.

Ultimately, the best way to determine which capitalization is correct is to consider the context and the intended meaning.