Usability testing is itself a big term. In usability 
testing you perform lots of test scenarios which a user can 
or cannot peroform in real world when he/she uses the 
product. It is a simulation of actual scenarios.

So it cannot be performed only manually or only by 
automation. Both are required in equal ratios.

Please let me know if it satisfies your query.

Jay prakash !! I understand where we can implement manual 
testing but in which scinarios we will use automation and 
which tool? Can usability testing be conducted by  QTP?

Hello Sameer,

Let me try to answer you:

>>"in which scinarios we will use automation "
Before any automation we generally do a feasibility study 
based on several factors. As the output of that study we 
list out the test cases / use cases which can or cannot be 
automated. The use cases identified for automation should 
be automated.

>>"which tool?"
again the tools can't be defined at this stage. that 
depends on your project needs. there are several automation 
tools available in the market. what suits best for your 
project you should use that. 

>>"Can usability testing be conducted by  QTP?"
Yes. QTP can also be used for the automation part for 
usability testing. But it is not the only tool. This 
depends on your project needs. Suppose u have to automate 
in linux than QTP can't help you.

Please let me know if i make myself clear.

Usability testing is done by manual testing not by 
Automation tools.How the application user friendly for end 
users identified oly through manual testing.Testers have to 
test the application in user aspect of view.

Understandable answer given by Jay Prakash, Thank you for 
your quick reply.

:)