China has agreed to invest $500m (£268m) in Cuba's nickel industry as leaders of the two communist nations met to forge closer relations.
Castro (r) is still largely immobile after a fall last month
The deal was one of several made during a visit to the island by Chinese President Hu Jintao.
"Cuba is one of China's largest commercial partners in Latin America," Mr Hu told gathered business leaders.
The agreement confirms the close ties between the two countries, strengthened since the fall of the Berlin Wall.
Mr Hu met Cuban leader Fidel Castro, still in a wheelchair after breaking his knee and arm in a fall in October, behind closed doors in Havana's Palace of the Revolution.
Mr Castro said the two countries shared the same ideal of socialism.
"Socialism will remain, in the end, the only real hope for peace and the survival of our species," he said, adding that China was an economic powerhouse and an example to all developing countries.
Relations between the two countries were tense during the Cold War, when the Caribbean island was closely allied with the former Soviet Union.
But since 1989, when Cuba lost its preferential trade and aid deals with the Soviet bloc, relations with China have improved.
The country is now the island's third trading partner after Venezuela and Spain, and represents some 10% of Cuba's foreign commerce.
Nickel is one of Cuba's key exports and a vital commodity for China's booming economy.
Since 2000, half of the country's nickel, used to make stainless steel, has come from Cuba.
Cuba's economy is currently facing difficulties following a decline in foreign tourism since 2002, periodic hurricanes and the increasing costs of importing oil.
Some 16 agreements, including deals on tourism and telecommunications, were overseen by Mr Hu on Monday.
The Havana visit is the last leg in President Hu's first trip to Latin America, which has taken him to Brazil, Argentina and Chile, where he attended an Asian Pacific Economic Cooperation summit.