The two leaders travelled together in a helicopter from Patna to Lakhisarai and Begusarai during the day.
Later, Kumar expressed happiness over the reunion with Lallan.
Addressing a function after inaugurating a RCC bridge over Kiul river at Sambho-Suryagraha in Lakhisarai district, he said "The day is important also because MP Rajiv Ranjan Singh Lallan is once again with us."
"People who move away comes near again, life's journey goes like this...Differences are over," a beaming Kumar said.
Lallan, once the most trusted aide of Kumar, had won the last Lok Sabha elections on JD(U) ticket from Munger but turned rebel before the 2010 assembly polls and indulged in anti-party activities by campaigning for Congress. JD(U) had initiated disciplinary action against him and suspended him from the party.
He was a petitioner against RJD president Lalu Prasad in the fodder scam and had played an important role in ending RJD rule and bringing in the NDA government under Nitish Kumar in Bihar in 2005.