(Corrects to show BOJ holds policy meeting this month, not next, in paragraph 5)
TOKYO, Sept 1 (Reuters) - Japanese government bonds prices slipped on Thursday, with 20-year bonds yield hitting a four-month high partly on speculation that the Bank of Japan may tweak its policy to effectively steepen the yield curve.
The 20-year yield rose 2.5 basis points to 0.355 percent , its highest level since late April, while the 30-year yield rose 3.0 basis points to 0.445 percent .
"The market is trying to price in a scenario that the BOJ will make its bond buying target flexible," said the head of derivatives trading at a Japanese brokerage.
The Bank of Japan's pledge in late July to conduct a "comprehensive assessment" on the impact of its stimulus measures has raised hopes among bond investors that the BOJ may finally heed their call to modify its heavy-handed stimulus.
A majority of economists expects the Bank of Japan will ease policy further this month, a Reuters poll found, while many respondents suspect a change in its inflation target terminology is likely.
Shorter maturities fared better on Thursday, supported by speculation that the Bank of Japan could consider cutting rates further into negative territory.
The five-year bond yield rose just 1.5 basis to minus 0.165 percent. The benchmark 10-year yield rose 1.0 basis point to minus 0.060 percent.
The three-month overnight indexed swap rate fell to one-month low of minus 0.1063 percent. The decline in the OIS rates accelerated after BOJ Governor Haruhiko Kuroda said on Saturday that he bank will approve more quantitative easing or lower negative interest rates "without hesitation." (Reporting by Hideyuki Sano; Editing by Kim Coghill)