Home Minister Kumar Waii and Tourism Minister Jarkar Gamlin and the six legislators were denied tickets by the BJP.
Waii said the BJP has lost its previous glory in the eyes of the people for its "false promises".
"We will not just fight the elections but will form an NPP government in the state," Waii said.
Besides the eight BJP MLAs, a legislator of the Peoples Party of Arunachal (PPA) and 19 other saffron party leaders also joined the NPP.
The NPP, which is ruling Meghalaya along with the saffron party as an alliance partner, has decided to field 40 candidates in the Arunachal Pradesh assembly polls which will be held simultaneously with Lok Sabha elections in the state.
NPP general secretary Thomas Sangma said, "We will go it alone in Arunachal Pradesh and announce the candidates list on Wednesday."
NPP state unit spokesperson Mutchu Mithi, however, said the party might go for post-poll alliance, if required.
The party would soon decide on fielding a candidate for the lone Lok Sabha seat in the state, Thomas Sangma said.
The NPP is a constituent member of the BJP-led North East Democratic Alliance (NEDA).