Q: In research, what is the basis difference between the rationale and the justification?

Actually, there is no difference between the rationale and the justification of the study. Both mean the same thing; the terms are used interchangeably. :-)

You may learn more about the rationale/justification of research through these resources:

• Can you give an example of the "rationale of a study"?
• How to write the rationale for research?
• What is the justification of a research?

As the rationale/justification is written in the Introduction of the paper, you may find this handbook helpful: How to write a strong introduction section that makes a great first impression

Hope that helps!