Would you please let me know why the restraints getting invisible when ZOOM IN?

What version and build are you running? Does the "Opengl/Windows Basic" video setting (in the Configuration) make a difference? Can you use the "fly-out" and change the restraint size?

I am using Ver 5.30.0 Build 101122
I tried what you mentioned no improvement

That is the original release of v.5.30. You've missed the following updates:

120525 - May 25, 2012 (D) 5.30.04 SP4 (released as 5.31)
120131 - January 31, 2012 (C) 5.30.03 SP3
110830 - August 30, 2011 (B) 5.30.02 SP2
110228 - February 28, 2011 (A) 5.30.01 SP1
Released - November 22, 2010 5.30.00

You should visit SmartSupport (formerly eCustomer) and download the latest update for this version (you are current on Maintenance right)?

SP3 did address a graphics issue where restraints may not draw correctly - this could be your problem.

It is solved, Thanks