[Wiiiiiiiiiiii]

t-butyl group is electron donating group, right ?

[psychoNOT]

Assuming you are talking about substituents on a benzene ring, alkyl groups, especially t-butyl are electron donating as a result of hyperconjugation.  If you aren't familiar with the term there is a good explanation on this website: http://www.chem.ucalgary.ca/courses/350/Carey/Useful/hyperconjugation.html.  Although, bulky alkyl groups like t-butyl will also have steric effects which will influence a reaction with a substituted benzene ring so that must also be taken into account.