I have confuse about these 2 words. I have translated them but they both have the same meaning in dictionary. Please make it be loud and clear for me.
Thanks.
I would not get too obsessed about the difference in meaning between these two words. The meanings are very similar.The meaning of words usually becomes clear to us through a significant amount of exposure. You know roughly what these words mean and you are probably not going to use these words very often. I would recommend to use the word āobsessiveā in situations where you have to use the word. Meanwhile you can just continue with the idea that the words mean the same thing.
While the two words are very similar in meaning, āobsessiveā is often used to describe a concern, a thought, an interest etc. āObsessionalā is more often used to describe a person, who might be exhibiting a psychological problem or anxiety in connection with an obsessive concern or interest in something. However both words are interchangeable.
So I suggest that an obsessive concern with the differences in meaning between these two words is not recommended, otherwise you might be considered obsessional by some people.
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English has a lot and a lot of synonyms!..
Sometimes thereās a little difference between them, but sometimes they are the same, for example:
resembling, similar, alike, like;
to stop, to cease, to end, to discotinue, to finish;
to postpone, to put off, to defer, to delay, to procrastinateā¦
I discuss about it with English native speakers in my collections in the English library of Lingq: āEnglish Grammar and Vocabularyā and āSecrets of English wordsā. You can read them if you would like to.
In this case āobsessionalā= āobsessiveā in the meaning: obsessional thoughts=obsessive thoughtsā
But āobsessiveā has an addional meaning, like here:ā She is obsessive about her clothes.ā
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Iām a native English speaker. I didnāt even know āobsessionalā was a word! How have I missed it?
Jingle, itās OK. English has the longest dictionnry in the world - more than one million words!.. (Russian has 500,000; German 450,000 and French 400,000).
Itās impossible to know all these words!..
And you neednāt to do it because we usually use in all our life not more than 10,000 words (and repeat them of course).
I would never use obsessional. My first thought was, āThat isnāt a word.ā Apparently it is. Still, Iād never use it.
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Thanks, Evgueny. 
Makes me wonder, though, where an English learner is coming across itā¦
Iād never heard of āobessionalā until now, too. To the OP, I would recommend not using that word since native speakers would be like, āyou aināt around these parts are you?ā and look at you funny!
Thanks everyone for giving feedback to me. You guys know, when I am interested in some new vocabulary, I will find out any kind of that word. For example, I have seen or heard the word āobsessā in the content that I read and listen to. So I will find the noun, adjective and abverb kinds of that word. And so! I saw those 2 words have the same meaning in the oxford dictionaryā¦
Donāt worry, I always study new vocabulary by remember and repeat the whole phrase or sentence, not individual word. Again, thanks everybody!
Ohhhā¦That explains it! Happy dictionary reading, quang! 
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