Combustion characteristics under various injection timings of a direct-injection engine fueled with natural gas-hydrogen blends at fixed injection duration and fixed ignition timing were investigated. The study shows that early injection decreases the excessive-air ratio and makes leaner mixtures. The brake mean effective pressure increases with the advancement of fuel-injection timings. The brake mean effective pressure reaches a maximum value at an injection timing of 190 CA BTDC and maintains this maximum value with the further advancement of fuel-injection timings. For a specific injection timing, an increase in the hydrogen fraction decreases the brake mean effective pressure when the hydrogen fraction is less than 10%, whereas the brake mean effective pressure tends to increase when the hydrogen fraction is larger than 10%. Combustion durations decrease with the advancement of fuel-injection timing. When the hydrogen fraction is less than 10%, combustion durations increase with increasing hydrogen fractions; conversely, combustion durations decrease with increasing hydrogen fractions when the hydrogen fraction is larger than 10%. The amounts of NOx and CO2 increase with advancing fuel-injection timing, and the CO concentration experiences small variations under various fuel-injection timings. The addition of hydrogen in natural gas can reduce the CO2 concentration.