Can any one help me in finding the city name in the map when I give zipcode through iPhone application.
As Deepak suggested, a Geocoding API is almost certainly the way to go. Google and Yahoo both provide good ones that can convert zipcodes into city names.
Here's a sample Google query for zipcode: http://maps.googleapis.com/maps/api/geocode/json?address=77048&sensor=true
And a sample Yahoo query for zipcode: http://where.yahooapis.com/geocode?q=77048&appid=0