In my problem, i have taken rgb image of size 512*512,then converted it to greyscale and applied dct2 on it.now i want to do huffman coding of this.Plz tell me how to do this and also tell me if i am wrong.

I=imread('lena.jpg'); % Reads the image whose name is lena & format is jpg.
whos                  % 
save I
load I
T1=I;
figure;imshow(I);
title('coloured image of lena')
T1=im2double(T1);
G=rgb2gray(T1);
whos
imshow(G);
imagesc(G);title('grayscale image of lena')
T2=G;
Z=dct2(T2);
figure;imshow(Z);
title('dct transformed')

[Walter Roberson]

dct is for lossy encoding, not lossless encoding.

dct and huffman is very likely "modified huffman encoding" rather than pure huffman encoding. You need to research the algorithm.