on/in somebody's mind ????

both are possible, with different meanings:

have sth on your mind
to be worrying about something:
Paul's got a lot on his mind at the moment.

have something in mind
to have a plan or intention:
Did you have anything in mind for Helen's present?

2007-01-31 20:59:58 補充：
You might also have heard of a classic hit called 「Always ON my mind」

2007-01-31 21:01:25 補充：
for more details please go to: http://dictionary.cambridge.org

2007-01-31 21:22:28 補充：
but if the verb is KEEP / BEAR, then you must say KEEP something IN MIND

2007-01-31 21:29:00 補充：
中文觲釋:我時刻都很掛念/憂慮她 --- She is always ON my mind.我已有了個主意 -- 下個月到美國見她. -- I have something IN mind already -- I will go to America to see her next month

be on one's mind / have sth on one's mind ( 使某人 ) 為某事物擔憂
eg :
My deputy has resigned , so I ' ve got a lot ( on my mind ) just now ,
我的副手辭職了 , 所以現在有許多事要我操心 ,

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in one's mind 表示某人的意識
eg :
There was no doubt ( in his mind ) that he ' d get the job ,
他毫不懷疑自己能夠獲得這份工作 ,