How to fix Normals

I am new to unity.I am loading .dae model file in the unity. my model has two parts. when i am seeing model from one side the one part is appearing and other not and when i am rotating the model the other part appears and the first one not.
I am not sure weather it is a normals problem or some other problem.

Please find the attached images.
If you want model i will mail you also.

Thanks You[|90614]

Both your description and your screenshot suggest that you’re right - this is a normals issue. i.e. some of the single-sided tris of your model are pointing the wrong way, so you’re looking at (or, rather, through) their back face rather than at their front face.

This is something you need to fix in your modelling program - either by manually flipping the incorrect faces, or using a calculate normals function.