I want to sort an array using NSSortDescriptor

Take a look here: Creating and Using Sort Descriptors

You can compare as case-insensitive.

NSSortDescriptor *sorter = [[[NSSortDescriptor alloc]
          initWithKey:@"w"
          ascending:YES
          selector:@selector(localizedCaseInsensitiveCompare:)] autorelease];
NSArray *sortDescriptors = [NSArray arrayWithObject: sorter];
[mGlossaryArray sortUsingDescriptors:sortDescriptors]; 

Just use NSSortDescriptor like I used and It worked fine.

   NSSortDescriptor * sortByRank = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES selector:@selector(caseInsensitiveCompare:)];

May I suggest using -localizedStandardCompare: (NSString)?

"This method should be used whenever file names or other strings are presented in lists and tables where Finder-like sorting is appropriate. The exact sorting behavior of this method is different under different locales and may be changed in future releases."

You may use this for sorting an array according to name thats also contains small letter:

NSSortDescriptor *sorter = [NSSortDescriptor sortDescriptorWithKey:@"w" ascending:YES selector:@selector(caseInsensitiveCompare:)];

NSArray *sortDescriptors = [NSArray arrayWithObject:sorter]; 

[mGlossaryArray sortUsingDescriptors:sortDescriptors];

This code work fine for me to sort name according to alphabets that has also small character i.e. rocky,Ajay,john,Bob etc.