Salut Malvinder,

You raise an interesting point.

The noun souris is both singular and plural depending on context.

So I suspect that both peur de souris or peur des souris could be correct where the translation is mouse or mice (mouses)

Looking forward to a staff expert's comment on this point.

Bonne continuation.

Jim

Good to learn that "souris" is both singular and plural. However, with this new knowledge I cannot really find a way that "Il a peur de souris" could be correct:

- If we interpret "souris" as singular:
We would need either a definite article ("He is afraid of the mouse (the one that just showed up)") that is "Il a peur de la souris" or an indefinite article ("He is afraid of a mouse!") that is "Il a peur d'une souris!".

- If we interpret "souris" as plural:
We need a definite article when we want to express "mice" in general ("He is afraid of mice"). Therefore, "Il a peur des souris" (contraction of de+les). If we want to express "some mice", I guess "Il a peur de souris" would work grammatically but I do not see how this would ever make sense ("He is afraid of some mice.").

Malvinder, 

as a supplement to Christian’s detailed answer, to understand the different uses of de/du/de la/des further, have a look at the link below to Laura Lawless site : 

https://www.lawlessfrench.com/grammar/de-vs-du-de-la-des-verbs/