- Go to Catalog -> Attributes -> Manage Attributes
- Click on SKU attribute
- Under Frontend Properties (on the first screen, at the very bottom) change Used for sorting in product listing to "Yes."
Here is how to do it on few categories you want.
- Go to Catalog -> Manage Categories.
- Click on the category you want to sort by SKU.
- Click on the Display Settings tab.
- Under Default Product Listing Sort by, uncheck the "Use Config Settings" and then select SKU from the dropdown.
- Go to System -> Configuration -> Catalog-> Catalog
- Under Frontend go to Product Listing Sort by
- Set SKU and Save Config