Is "learning" part of cognitive psychology? in Wikipedia, 


  Cognitive psychology is the study of mental processes such as "attention, language use, memory, perception, problem solving, creativity and thinking."


I wonder if learning is also a mental process that cognitive psychology studies, although it is not mentioned in Wikipedia? Thanks!
Yes, cognitive psychology incorporates the study of learning. You can see this reflected in other sources that provide general descriptions of cognitive psychology, e.g.:


http://psychology.about.com/od/cognitivepsychology/f/cogpsych.htm
http://psych.rutgers.edu/co
http://infed.org/mobi/the-cognitive-orientation-to-learning/


I'm not recommending these as especially definitive sources; these are just the other three of the first four hits for a Google search of "cognitive psychology learning".

However, cognitive perspectives on learning are not the only perspectives available in psychological theory (see also radical behaviorism).