As I know, generally, the JPEG is consist of DCT, Q ,Huff.

But I'm not sure which one is lossy? all of them? of just Q?

I think Q is lossy but else not. I just want to know that.

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    \$\begingroup\$ I'm voting to close this question as off-topic because it appears to be a general image compression question with no EE specific content. \$\endgroup\$ – PeterJ Jun 11 '15 at 15:22

The quantization stage is the lossy one, a number of bits is assigned to a certain DCT coefficient. The ones which are not noticeable to the human eye are assigned the least amount of bits.

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