It was Kumar's first visit to the national capital after he jettisoned the alliance with the RJD and the Congress, and sealed a tie-up with the BJP on July 26.
He told reporters after meeting Modi that it was a courtesy call and he will visit Delhi again this month end to discuss Bihar's developmental issues with the prime minister.
JD(U) sources said the party is expected to announce its decision to formally join the BJP-led NDA in its national executive meeting on July 19 in Patna.
Asked if the party will also join the Modi cabinet, a JD (U) leader said such a decision will be "natural".
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"When we are together in government in Bihar it is only natural that our party joins the Union government," he said.
However, he added that such a decision will be taken by Kumar and the party had not deliberated over it yet.
Its leader in the Rajya Sabha Sharad Yadav has rebelled against the alliance with the BJP but apparently does not enjoy much support among other party's lawmakers.
Kumar almost shut the door on any reconciliation with Yadav, saying he is free to take any decision as the alliance with the BJP had the entire party's nod.
"He (Yadav) is free to take his decision. As far as party is concerned, it has already taken its decision. The decision was not mine alone and it was taken with the consent of the party. If he keeps a different opinion, then he is free to do so," Kumar said.