Memphis-based International Paper Co. on Thursday reported its best year in nearly 20 years with net earnings of $1.3 billion compared to $644 million in 2010.

“This result marks the best year that we’ve had financially in almost two decades,” said IP chief financial officer Carol Roberts.

The earnings amounted to $3.07 diluted earnings per share for the year, compared to $1.48 in 2010. For fourth quarter 2011, the diluted earnings per share were 59 cents compared to 73 cents in the last three months of 2010.

The paper company posted annual sales of $26 billion, which was $800 million better than 2010. But for the last quarter, the net sales of $6.4 billion were about $100 million less than fourth quarter 2010.

Roberts attributed the quarterly numbers to “the normal seasonal weaknesses in North America.” Some of the “headwinds” are caused by maintenance at plants that took them offline.

The company has more coming up in the second quarter in its container board plants, which will lead to some increases in production this quarter to have enough inventory on hand to compensate for the temporary closings for maintenance.

IP’s annual operating profits, including special items, jumped $500 million from 2010 to 2011 with a 2011 figure of $2.2 billion.

The operating profits also increased for the fourth quarter at $577 million, compared to $561 million in 2010’s last quarter.

Chairman and CEO John Faraci told analysts on an earnings call that the company’s multi-year re-examination of what businesses it is in and how it operates was completed in 2011.

“The transformation plan is over,” he said. “With exception of 2009, we’ve steadily been on the march.”

While the rethinking of IP’s structure is complete, carrying out the deliberate moves is not.

Faraci said International Paper executives have now completed their visit and assessment of Temple-Inland mills and based on what they’ve seen, are more certain that the coming merger of the companies will result in $300 million of “synergies” or savings from efficiencies.

IP’s $4.3 billion acquisition of Austin, Texas-based Temple-Inland was the major event of 2011 for International Paper.

The company announced in January it is extending its due diligence deadline to Feb. 13 to better answer U.S. Justice Department antitrust law questions about the merger. The regulatory approval is necessary for the merger to begin.

IP also recently opened the gates on its Franklin, Va., fluff pulp mill for the first time in two years to begin taking delivery of wood to be used in the renovated mill.

The $83 million repurposing of the mill adds to IP’s status as the third-largest global supplier of fluff pulp, which is used in products ranging from baby diapers and baby wipes to cleaning and feminine hygiene products.

The pulp is made from U.S. southern pine.

The Franklin plant is expected to open in late summer and create 200 jobs in a plant that will produce its own energy from renewable biomass.