Are all grasses monocots ?

Yes, monocots have parallel veins and so do all grasses.

http://wiki.answers.com/Q/Is_grass_a_monocot

Traditionally speaking, yes, all grasses are monocots. Monocots are grasses and grass alternatives. However, most scientists agree that the division between monocot and dicot is a fuzzy one. Many plants have characteristics of both because both categories of plants share the same ancestors. Check out both of these sites.

https://www.diffen.com/difference/Dicot_vs_Monocot
www.backyardnature.net/monodico.htm

Monocot is short for monocotyledon, meaning one seed leaf. All grasses, having only one seed leaf at germination, are then monocots.

From Wikipedia entry for monocotyledons:
(mid PP2) "The true grasses, family Poaceae (Gramineae), are the most economically important family in this group."

https://en.wikipedia.org/wiki/Monocotyledon

But the reverse is not true. All Monocots are not grasses. Many orchids germinate with a single seedleaf.