Kenneth V. Duncan, Prince George's County chief administrative officer and the county's highest-ranking administrator, announced yesterday that he will leave county government, probably within the next six months. County Executive Parris Glendening, who took office last month, called Duncan "the single most knowledgeable person in county government" and said he regretted Duncan's decision.

Duncan, 44, said he made the announcement as a courtesy to Glendening, because he has begun searching for other work and is considering at least three job offers, none of which he would name. Duncan said he wanted to give Glendening time to plan for his departure from the $67,500-a-year post, and did not wish Glendening to hear of his plans secondhand.

"I did not feel too good about being in discussion with people, some of whom Parris might know, that would get back to Parris in a roundabout way. I just wanted to be up front about it," said Duncan. Two of his offers are in the private sector, he said, and all are in the metropolitan Washington area.

Duncan, a former newspaper reporter from Harford County, has served 11 years in Prince George's county government. Former County Executive Lawrence J. Hogan, a Republican, selected Duncan to be his chief administrative officer in 1978. While in the job, Duncan earned a reputation as an extremely hard-working official whose distaste for publicity was a stark contrast to Hogan's apparent love for the limelight.

Duncan said he made clear to Hogan upon his appointment that he would not serve more than four years.

Glendening said he had not solicited Duncan's resignation and had, at various points before his election, asked Duncan to continue, but Duncan refused.