I'm no DB2 person, but
1) Why would you not think it possible -- as long as a single column consists of unique values, a unique index on that column makes sense
2) Even I know that a unique index can be built on a table without a primary key, so obviously the answer is yes
Dear Debasis Apart from what Robert and Gnanas had already explained in their post
there are a few more differences exists between Unique index and Primary Index.
You can have as many as (practically beyond the range of an application program ) Unique indexes in a table but only one Primary index. Primary index in the sense - index created on primary key.
For applying Referential Constraints between tables, you must have a primary key and primary index on the parent table, but unique index will not help. Unique index will help to make the value in the associated column as unique.