U.S. stocks opened mildly up on Monday, boosted by a rally of Japanese stocks and lingering positive sentiment among investors after last week\'s U.S. non-farm payroll data. The 225-issue Nikkei Stock Average closed up 636.67 points, or 4.94 percent on Monday, marking the biggest point gain since October 2008, as a tepid U.S. non-farm payroll survey in May fueled investors\' expectation that the U.S. Federal Reserve will maintain its quantitative easing for the time being. The U.S. non-farm payroll employment increased by 175,000 in May, and the unemployment rate ticked up 0.1 percentage point to 7.6 percent in May, according to data released last Friday by the U.S. Labor Department. The market was also underpinned by an upgrade of the U.S. government\'s rating outlook by the Standard & Poor\'s Ratings Services on Monday. The rating agency revised up its outlook on the U.S. long-term rating to stable from negative, saying the move indicated its \"current view that the likelihood of a near-term downgrade of the rating is less than one in three.\" However, a sharp decline in China\'s exports in May capped Wall Street\'s gain. China\'s exports in May inched up merely 1 percent year on year in May from a 14.7 percent increase in April, while imports decline 0.3 percent in May from a 16.8-percent gain in the prior month, according to official data released over the weekend. With no major U.S. data due on Monday, investors are keeping an eye on a speech by St. Louis Fed President James Bullard on global economy. The Federal Reserve will hold its monetary policy meeting next week. Shortly after the opening bell, the Dow Jones Industrial Average rose 22.56 points, or 0.15 percent, to 15,270.68 points. The S&P 500 edged up 2.44 points, or 0.15 percent, to 1,645.82 points. The Nasdaq Composite Index added 2.93 points, or 0.08 percent, to 3,472.15 points.