Trie for lowercase words
Do I have to replace the fixed sized array of object of class Dict with any other technique?


I suggest making the size a constant, like this:

public static int ALPHABET_SIZE = 26:


And then you may use:

obj = new Dict[ALPHABET_SIZE];


As you can see, the comment now becomes redundant and we can remove it, and it is always a good thing, as comments may become obsolete or wrong.

The same applies to:

int Linkno=((int)(str.charAt(it)))-97; //97 is the ascii value of 'a'.


Make a constant:

LOWER_A_ASCII_VAL = 97;


Get rid of repetition

Looking at:

    root.insertWord(root,"trasaction",0);
    root.insertWord(root,"transformation",0);
    root.insertWord(root,"transmission",0);
    root.insertWord(root,"trance",0);
    root.insertWord(root,"trap",0);


What if you want to add 10 more words? The code will become very long and repetitive, instead use a foreach loop:

for (String word : {"transaction", "transformation", ...} ) {
    root.insertWord(root, word, 0);
}


Miscellanous


Wrong indent hurts readibility, fix it in:

    if(it&lt;str.length())
    {
    int Linkno=((int)(str.charAt(it)))-97; //97 is the ascii value of 'a'.
    if(Node.obj[Linkno]==null)
    {
        Node.obj[Linkno]=new Dict();
    }
    insertWord(Node.obj[Linkno],str,it+1);
    }

Always use braces to prevent bugs in :

        if(Node.obj[i].isWord==true) {
            System.out.println(str+((char)(i+97)));
        }

Avoid general names like obj and think about a more precise name.