"The eighteen major Puranas are the Brahma, Padma, Vishnu, Shiva, Linga, Garuda, Narada, Bhagavata, Agni, Skanda, Bhavishya, Brahma-vaivarta, Markendeya, Vamana, Varaha, Matsya, Kurma and Brahmanda Puranas."
In a conversation with his disciples, Shrila Prabhuapada tells about the importance of the Padma Purana:
Srila Prabhupada: "So, there are
different kinds of Puranas,
eighteen Puranas ?but, t he
sattvika,-puranas (those in
goodness)-they are meant for
immediately worshiping the
Supreme Personality of
Godhead??So Padma Purana is one
of the Puranas in the mode of
goodness, passion, and
ignorance, and to reclaim all
these conditioned souls, there
are eighteen Puranas. Six
Puranas are meant for those in
the mode of goodness, six for
those in the mode of passion,
and six for those in the mode of
ignorance. The Padma Purana is
written for those in the mode of
goodness."

