Sina Corp, the largest online portal in the country, reported a 73 per cent increase in profit, fuelled by strong brand advertising growth and stable wireless revenue.

Net profit for the first quarter reached US$19.6 million, as revenue grew 39 per cent to US$71.3 million from a year earlier. Non-GAAP diluted earnings per share were 33 US cents, which were ahead of consensus estimates of 27 US cents.

Sina forecast second-quarter revenue would be even better at US$88 million to US$90 million, or an about 47 to 50 per cent increase from a year earlier, higher than the consensus estimate of US$82.1 million.

Shares of the Nasdaq-listed firm soared 11 per cent to US$59 in after-hours trading following the results announcement.

"For a long time, the performance of Sina has been mediocre," said Jason Brueschke, the head of Asia internet/media research at Citigroup. "This is disappointing given the strong growth in China's internet and Sina's powerful position in the market. But in the first quarter, we have seen the investment the company made in the last few years finally paying off."

Brand advertising, which accounted for 67 per cent of the Beijing firm's total revenue, soared 51 per cent from a year earlier, much more than the usual 40 per cent annual rate. In the past two years, Sina has invested heavily in its finance, vehicle and property sections.

The three channels delivered 80 per cent of Sina's online advertising growth. Mr Brueschke expects the growth to continue. But since the investment required to build content is high, he said profit margins would not improve until the fourth quarter.

Wireless services - which have dragged down Sina's margin since China Mobile introduced its user protection policy in 2006 - also showed significant improvement.

"After a year of decline, the wireless business has bottomed out," said Mr Brueschke.

Sina's wireless revenue has been growing in the past two quarters. It reached US$21.7 million in the first quarter, representing an increase of 19 per cent from a year earlier and 16 per cent from the previous quarter.

"The wireless sector has been stabilised. However, it would never have margin and growth rates as high as before," said Dick Wei, a China internet analyst at JP Morgan.

The company expects 50 per cent revenue growth this year. But most analysts believe the growth rate will slow down after the Olympics. Mr Wei said the online advertising market would grow 20 to 30 per cent next year, down from the 30 to 40 per cent range of the past few years.