Q: How to report a very small p-value?

In case of very small p-values, the convention is to write it as p<0.001. Some journals mention this in the author guidelines. For example, the New England Journal of Medicine (NEJM), also states that p-values smaller than 0.001 should be reported as P<0.001.

The manual of the American Psychological Association (APA), which is one of the most often used citation styles, states (p. 139, 6th edition) also recommends using p < 0.001.  Thus, if a p-value is 0.00000002 or any such small number, it should be written as p<0.001.

You refer the following article for more information on how p-values should be reported:

•  The correct way to report p values