[mimimomo]

Here it is from petplace:
We're not sure why your pup likes grass, although there have been many theories offered. Primarily, dogs are descended from wild canids (wolves and foxes), which ate the entire "kill" when they hunted for food. Since they consumed many herbivores (plant-eating animals), they wound up eating a lot of plants and even berries found in the stomach and intestines of their prey.
Interestingly, carnivores tend to go for the stomach and its contents first, so it's likely that dogs may eat grass because they like it and it was once part of their normal diet.

Then there is the great mystery: Do dogs eat grass to make themselves vomit? Or do they vomit because they eat the grass? Most veterinarians believe that dogs eat grass simply because they like it, and vomiting just naturally follows. When dogs eat grass, the grass acts as an irritant and causes vomiting. However, they may not be smart enough to use grass as a medicine when they have an upset stomach. That said, one of us has seem a dog with burrs stuck in his throat pounce on a Ficus plant and voraciously start scoffing great scads of leaves until he threw up, presumably in an attempt to dislodge the foreign material.

Some veterinarians believe that dogs eat grass because their prepared diets are lacking in greens and so they eat grass. And as some support of this contention, dogs sometimes seek out a particular variety of grass to nibble.

No matter what the reason, your dog's "grass" habit is normal behavior and you need not be concerned about it. A note of caution, however: Take care that your pet does not eat grass that has been treated with fertilizer, or herbicides. This could cause stomach upset or even worse problems for your dog.