Yes it is possible to decompile an apk .
Depending on obfuscation level it might take some time but a dedicated/bent person will eventually decompile it .
You can try tools like
apktool
Smali
Show Java Android App
Source : http://geeknizer.com/decompile-reverse-engineer-android-apk/
UPDATE 1
Here are 2 more options for you:
decompileandroid.com
javadecompilers.com/apk
suggested by @AndrewRukin
UPDATE 2
Another tool : jadx
