@stampede - I believe you haven't got the problem I am actually facing. Sorry, i couldn't explain my problem clearly. The parent item i am talking about is the QGraphicsLineItem which I dropped on the QGraphicsScene, the parent line item is able to rotate and resize on the graphics scene. This is the one i created last week and we did discuss rotation/resize on this line.

Now the Arrow item I am referring to is actually the combination of the line item + the arrow head( i used trignometry to create arrow head). I used parent line's normal vector to create a arrow line perpendicular to the parent line. Initially the arrow line's P1 has the same origin(0,0) as the parent item. But then I shifted the arrow line to some random point which is QPointF(-5.0, arrow_line_start_point.y()). The arrow_line_start_point is nothing but parent_line.pointAt(0.5). My requirement is that the arrow item(combination of arrow line + arrow head ) should actually be positioned in such a way that, it should intercept the parent line item. The arrow item once positioned at some point, should maintain its relative position when I rotate the parent graphics line item on scene.
When ever I rotate the parent line item the arrow item should also rotate along with the parent line item and also should remain perpendicular to the parent item. I have attached the drawing of relationship b/w the parent line and arrow item.

Your were offering a possible exaplantion on creating arrow head to a arrow line which is a normal vector of parent line. My focus is currently more on transformations, when I am rotating the parent item the child arrow item is unable to maintain its relative position. How should i solve this problem?