The vote is meant to stabilise Ukraine after mass street protests toppled Moscow-backed president Viktor Yanukovich in February, but pro-Russian separatists have vowed to prevent the poll going ahead in eastern towns where they have seized control.
"I expect Russia to respect the OSCE's assessment, which will without doubt be objective; after all it belongs to this organisation itself," she said according to an advance copy of an interview to be published in Saarbruecker Zeitung on Friday.
About 1,000 observers from the OSCE, Europe's leading human rights and democracy watchdog, will spread across Ukraine, an ex-Soviet republic with a population of 46 million, for Sunday's poll.
Merkel said it would be one of the OSCE's biggest ever electoral observation missions. "Unfortunately Russia has refrained from also sending observers on the OSCE mission even though it was invited to do so," she added.
Russian President Vladimir Putin, who has condemned the overthrow of Yanukovich as a coup, continued to cast doubt on the legitimacy of Ukraine's election on Wednesday. He said during a state visit to China it would be more logical for Kiev to hold the vote after a referendum on a new constitution.
Merkel reiterated that the election was vital for the stabilisation of the country.
"What we're all seeing is that the Russian leadership obviously finds it hard to accept that the sovereign country of Ukraine is going its own, self-determined way," she said.
"Our view is that Ukraine should and can choose freely what it wants."
Merkel said she spoke regularly with Putin and added that they would be able to see each other during ceremonies to commemorate the 70th anniversary of the Allied landings in Normandy on June 6.