Hi,

When some content is assigned to the siverlight richtextbox , all the content is displayed with red colored underline even for most frequently words like a, in, an, the, this, for etc..... Is there any way to load default system dictionary instead of custom dictionary to avoid these underlines?

Hello Deepthi,

The spell checking in RadRichTextBox is enabled by default. You have control over it through the property IsSpellCheckingEnabled. 
The dictionary that it uses is located in Telerik.Windows.Documents.Proofing.Dictionaries.En-US and you should add a reference to the assembly in order to load the dictionary. It is usually discovered through MEF, however, if you are using library caching, MEF will not be able to load the assembly, but you can proceed as described in this forum thread.
If that is not the case and you do not manage to get spell checking working properly, we would greatly appreciate some more information on your scenario and implementation. Preferably, a demo project.

All the best,
Iva
the Telerik team