I drew objects in model space and dimension them in paper space. I found the problem is sometimes the dimensions change to zero autometically without any changes in the objects. How can it happen?

I am not sure what you are trying to do here, Why do you want to dimension in paperspace? by creating a dimension layer in model space you can control how these are seen and scaled in paperspace, so...what end result are you trying to get to?

it is better to dimensions objects in the same space. When drawing in model and trying to dimension in paper, dimensions are not attached to the object so it is easy for the dimension to move if you inadvertently move them.  

I try to use one object in model space in many viewports and many paperspaces and also different scale. I feel dimensioning them in paperspaces is easier and make model object clean.

msplcdykee69: When I dimension the model space objects in paperspace, if I snap to the objects' point the dimension line will attached to the objects. When I resize the objects, the dimension line will update itself. Just don't understand sometimes I'm sure I haven't modified the objects but dimension line go to zero!? Do you think it's better to use Annotation?

I use Annotation dim styles as well as text styles myself; I wind up with different viewports at different scales as well. It is one less thing I have to keep up with when setting drawings up in my line of work.