Might be helpful for you:

http://boards.developerforce.com/t5/Visualforce-Development/Aggregate-Relationship-is-used-in-an-unsupported-complex/m-p/275441

But remember that if you are rendering page as PDF then iframe will not work.

Thanks for reply.........

Am also getting same error

Error: Aggregate Relationship is used in an unsupported complex expression containing 'opportunity__r.opportunitylineitems'

am rendering paga as PDF, Is there any way to solve this error?...by writting apex code like that

Ahh, you can write apex class? Then there is hope :)

In the constructor, 

public class UrController {
 Cust_c custObj;
 List<OpportunityLineItems> lstOLI {get; set}
public UrController(ApexPages.StandardController controller) {
 custObj = (Cust__c) controller.getRecord();
 lstOLI = [select name,field1,field2... from OpportunityLineItems where Opportunity =: custObj.Opportunity__c];
}
}

Now use lstOLI in your VF page's repeat tag to get OpportunityLineItem..

When i used your code, i got error message as OpportunityLineItem is cant used, then I used the following query to solve this error.

SELECT Amount, Id, Name, (SELECT Quantity, ListPrice,
  PricebookEntry.UnitPrice, PricebookEntry.Name
  FROM OpportunityLineItems) FROM Opportunity

Then in the visualforce page, i used 'lstOLI' in the repeat tag, after saving the VFpage am getting the following error

Error: Unknown property 'CustObjectA__cStandardController.$lstOLI 



Please post your VF and controller code. I can assist you better..

And the select statement should be something like,

lstOLI = [select name,field1,field2... from OpportunityLineItem where OpportunityId =: custObj.Opportunity__c];

This is my Contoller class.....

public class CustomInvoiceExtension{
 Invoice__c custObj;
 List<Opportunity> lstOLI {get; set;}
public CustomInvoiceExtension()
{
}
public CustomInvoiceExtension(ApexPages.StandardController controller) {
 custObj = (Invoice__c) controller.getRecord();
 lstOLI = [SELECT Quantity, ListPrice,PricebookEntry.UnitPrice,PricebookEntry.Name FROM OpportunityLineItems Where OpportunityId =: custObj.Opportunity__c];
}
}

While saving am getting following error

Error: Compile Error: sObject type 'OpportunityLineItems' is not supported. If you are attempting to use a custom object, be sure to append the '__c' after the entity name. Please reference your WSDL or the describe call for the appropriate names. at line 9 column 11

Note:-

In my VF page, am using StandardContoller as my Custom object and Extensions this Contoller

Hey use:

lstOLI = [SELECT Quantity, ListPrice,PricebookEntry.UnitPrice,PricebookEntry.Name FROM OpportunityLineItem Where OpportunityId =: custObj.Opportunity__c];

Still same eror???

When they say refer WSDL just go to Setup--Develop--API--Generate Enterprise WSDL--Generate. Search for OpportunityLineItem and see the name used in database.