Hi Shiva Rajendran,

I had make some changes in below coding now it's working as well.. 
 

trigger QuoteLineItemUpdateOnOpp on Quote (after insert,after Update) {
    IF(Trigger.isUpdate && Trigger.isAfter){
        
        System.debug('After Update : ');
    Set<ID> qtId = New Set<Id>();
    Set<Boolean> syn = New Set<Boolean>();
    Set<Id> OppId =  New Set<Id>();
    List<OpportunityLineItem> lItem = New  List<OpportunityLineItem>();
   Set<Id> setPrId  = New Set<Id>();     
   Map<Id,Id> PrMap  = New Map<Id,Id>(); 
        
    for(Quote qId : Trigger.new){
        if( Trigger.oldMap.get( qId.Id).Custom_Synchronize__c != qId.Custom_Synchronize__c ){        
        qtId.add(qId.Id);
        syn.add(qId.Custom_Synchronize__c);
        System.debug('Old Map Id: ' + qId.Id);
         
    }
        }
       
    //Quote qt=trigger.new[0];
    //Quote qt = New Quote();
    IF(qtId.size() > 0){
        System.debug('Quote Size : ' + qtId.size());
       // List<Quote> qtList = [Select id,name from Quote Where ID =:qtId];
        List<Quote_Line_Item__c> qtLItem = [Select Id,name,Quantity__c,Price__c,Quotes__c,Quotes__r.Opportunity.Id, Products__c from Quote_Line_Item__c
                                           Where Quotes__c IN :qtId];
        System.debug('Quote Line Item Size : ' + qtLItem.size());
        System.debug('Checking a Quote LineItem : ' + qtLItem);
        
        for(Quote_Line_Item__c qLi : qtLItem){
            setPrId.add(qli.Products__c);
        }
        
        for(PriceBookEntry pEntry : [Select id,Product2Id from PriceBookEntry Where Product2Id IN :setPrId AND IsActive = True LIMIT 1]){
            PrMap.put(pEntry.Product2Id, pEntry.Id);
        }
         
        //List<Opportunity> oppty = New List<Opportunity>();
        List<Opportunity> oppList = [Select id,name,CloseDate,(Select id,name,ListPrice from OpportunityLineItems) From Opportunity Where ID IN :oppId];
        
        //Update QuoteLineItem To OpportunityLine Item
              
        
        For(Quote_Line_Item__c qteLineItem  : qtLItem){
            OpportunityLineItem OppLItem = New OpportunityLineItem();
            
            //OppLItem.Id = qteLineItem.Quotes__r.id;
            OppLItem.OpportunityId = qteLineItem.Quotes__r.Opportunity.Id;
            OppLItem.UnitPrice = qteLineItem.Price__c;
            OppLItem.Quantity = qteLineItem.Quantity__c;
            OppLItem.PricebookEntryId = PrMap.get(qteLineItem.Products__c);
            lItem.add(OppLItem);
        }
        
        if(lItem.size() > 0){
            
            insert lItem;
        }
        
    } 
}   
    
}

Quote ---> QuoteLineItem (Related List)---> Product will be create in Opportunity --> OppLineItem (Related List) When we click a synchronous button ....  


Thanks for help me ...

Regards,

Soundar Rajan P