On entering USA as a F41 immigrant visa holder, the immigration officer stamped my passport and verbally told me that it will be valid for 1 year (during which PR card will processed) but he didn't wrote any validity date. Now i have to travel in and out of USA. Do i need to be concerned and what Should be done in this case?
When you first enter the US on an immigrant visa, the immigration officer will stamp your passport and visa together, so that the stamp is partly on the passport page and partly on the immigrant visa. This is called an endorsement.
The endorsement stamp activates the clause printed on your visa that states: "UPON ENDORSEMENT SERVES AS TEMPORARY I-551 EVIDENCING PERMANENT RESIDENCE FOR 1 YEAR."
Thus, it's not necessary to write a validity date on the passport stamp. Your stamped visa is good for re-entry for one year, and is evidence that you are a permanent resident. An "admit until" date would be senseless.
During that one year period, you may enter the US as a lawful permanent resident using the endorsed visa, as many times as you wish, exactly the same as the permanent resident card you will soon receive. You may use the immigration lanes designated for lawful permanent residents, at the ports of entry where they are available.
(And note that the endorsement stamp is valid even if it is stamped next to the visa, rather than across it.)