how to create a block of 5x5 pixel in image

edit

By using a region of interest. More info on this can be found here. Scroll a bit down and start reading :)

I dont want to create matrix. i m already having a mat image, i want to select its 5x5 pixel from origin. and then want to find local standard deviation of it. like stdfilt() function

Well then again, if you had read correctly you could see that there is a region of interest parameter that you can use. Basically what you then do is:

 Mat existing_image = ... ; //defined by you
 Mat local_region = existing_image( Rect(x,y,w,h) ); //adjust parameters to your needs - creates an extra header only, not a deep clone, so it is the same data!
 applyfunction(local_region);

That will do the trick for you...