inventory<>0 but Unit cost=0 is this right???

If you are talking about item card, then it is possible that the item transaction may be posted without any cost.

Thus way the quantity can be more that zero without any Unit Cost.

There is no point correct or incorrect. What matters is why you want to do so??
Anyhow if it has been done, then you can use the revaluation journals to update the correct unit cost from the original transaction posting date.

In this case, if you revalue the item at its original cost(which has not been entered at the time of posting the positive transaction) on its original positive transaction date, and then run the batch job "Adjust Item-Cost entries", the system will itself assign the cost amount actual in the value entry created at sale.

Check by posting a transaction. Thus, it shows in statistics or not, the cost is properly assigned.